Narrative of a Survey of the Intertropical and Western Coasts of Australia Volume II Part 60

(**Footnote. Captain King informs me that the soundings in this part of the coast bring up a very fine quartzose-sand like that cemented in the breccia.)

ROTTNEST ISLAND, about four hundred and fifty miles south of Dirk Hartog's Island. Indistinct specimens containing numerous fragments of sh.e.l.ls, in a calcareous cement; the substance of these sh.e.l.ls has at first sight the appearance of chalcedony, and is harder than ordinary carbonate of lime.

The characters of the sh.e.l.ls in Captain King's specimens from this place are indistinct; but the specimens at the Jardin du Roi, which, there is reason to suppose, have come from this part of the coast, contain sh.e.l.ls of several species, belonging among others to the genera, corbula, chama, cardium, porcellanea, turbo, cerithium. M. Prevost, to whom I am indebted for this account, observes that notwithstanding the recent appearance of the sh.e.l.ls, the beds which contain them are stated to occur at a considerable height above the sea: and he remarks that the aspect of the rock is very like that of the sh.e.l.ly deposits of St. Hospice, near Nice.

KING GEORGE'S SOUND, on the south coast, east of south from Cape Leeuwin.

Beautifully white and fine quartzose sand, from the sea-beach. Yellowish grey granite, from Bald-head. Two varieties of a calcareous rock, of the same nature with that of Dirk Hartog's Island; consisting of particles of translucent quartzose sand, united by a cement of yellowish or cream-coloured carbonate of lime, which has a flat conchoidal and splintery fracture, and is so hard as to yield with difficulty to the knife. In this compound, there are not any distinct angular fragments, as in the stone of Dirk Hartog's Islands; but the calcareous matter is very unequally diffused.

A third form in which this recent calcareous matter appears, is that of irregular, somewhat tortuous, stem-like bodies, with a rugged sandy surface, and from half an inch to an inch in diameter; the cross fracture of which shows that they are composed of sand, cemented by carbonate of lime, either uniformly mixed throughout, or forming a crust around calcareous matter of a spongy texture; in which latter case they have some resemblance to the trunks or roots of trees. A ma.s.s, which seems to have been of this description, is stated to have come from a height of about two hundred and fifty feet above the sea, at Bald-head, on the South Coast of Australia. These specimens, however, do not really exhibit any traces of organic structure; and so nearly resemble the irregular stalact.i.tical concretions produced by the pa.s.sage of calcareous or ferruginous solutions through sand* that they are probably of the same origin; indeed the central cavity of the stalact.i.te still remains open in some of the specimens of this kind from Sweer's Island in the Gulf of Carpentaria. The specimens from Madeira, presented to the Geological Society by Mr. Bowdich, and described in his notes on that island,**

appear upon examination to be of the same character. But there is no reason to suppose that the trunks of trees, as well as other foreign substances, may not be thus incrusted, since various foreign bodies, even of artificial production, have been so found. Professor Buckland has mentioned a specimen of concreted limestone from St. Helena, which contains the recent sh.e.l.l of a bird's egg;*** and M. Peron states that, in the concretional limestone rock of the South Coast of New Holland, the trunks of trees occur, with the vegetable structure so distinct as to leave no doubt as to their nature.****

(*Footnote. Tubular concretions of ferruginous matter, irregularly ramifying through sand, like the roots of trees, are described by Captain Lyon as occurring in Africa. Lyon's Travels Appendix page 65.)

(**Footnote. Excursions in Madeira 1825 page 139, 140; and Bull. des Sciences Naturelles volume 4 page 322.)

(***Footnote. Geological Transactions volume 5 page 479.)

(****Footnote. Peron 2 page 75.)

INSTRUCTIONS FOR COLLECTING GEOLOGICAL SPECIMENS.

It so often happens that specimens sent from distant places, by persons unpractised in geology, fail to give the instruction which is intended, from the want of attention to a few necessary precautions, that the following directions may perhaps be useful to some of those, into whose hands these pages are likely to fall. It will be sufficient to premise, that two of the princ.i.p.al objects of geological inquiry, are, to determine, first, the nature of the MATERIALS of which the earth is composed; and, secondly, the relative ORDER in which these materials are disposed with respect to each other.

1. Specimens of rocks ought not, in general, to be taken from loose pieces, but from large ma.s.ses in their native place, or which have recently fallen from their natural situation.

2. The specimens should consist of the stone unchanged by exposure to the elements, which sometimes alter the characters to a considerable distance from the surface. Petrifactions, however, are often best distinguishable in ma.s.ses somewhat decomposed; and are thus even rendered visible, in many cases, where no trace of any organized body can be discerned in the recent fracture.

3. The specimens ought not to be too small. A convenient size is about three inches square, and about three-quarters of an inch, or less, in thickness.

4. It seldom happens that large ma.s.ses, even of the same kind of rock, are uniform throughout any considerable s.p.a.ce; so that the general character is collected, by geologists who examine rocks in their native places, from the average of an extensive surface: a collection ought therefore to furnish specimens of the most characteristic varieties; and THE MOST SPLENDID SPECIMENS ARE, IN GENERAL, NOT THE MOST INSTRUCTIVE.

Where several specimens are taken in the same place, a series of numbers should be added to the note of their locality.

5. One of the most advantageous situations for obtaining specimens, and examining the relations of rocks, is in the sections afforded by cliffs on the seash.o.r.e; especially after recent falls of large ma.s.ses. It commonly happens that the beds thus exposed are more or less inclined; and in this case, if any of them be inaccessible at a particular point, the decline of the strata will frequently enable the collector to supply himself with the specimens he wishes for, within a short distance. Thus, in Sketch 4, which may be supposed to represent a cliff of considerable height, the observer being situated at a, the beds b, c, d, though inaccessible at that place, may be examined with ease and security, where they successively come down to the sh.o.r.e, at b prime, c prime, and d prime.

6. To examine the interior of an unknown country, more skill and practice are required: the rocks being generally concealed by the soil, acc.u.mulations of sand, gravel, etc., and by the vegetation of the surface. But the strata are commonly disclosed in the sides of ravines, in the beds of rivers and mountain-streams; and these, especially where they cross the direction of the strata, and be made, by careful examination, to afford instructive sections.

7. Among the distinctive circ.u.mstances of the strata, the remains of organized bodies, sh.e.l.ls, corals, and other zoophytes, the bones and teeth of animals, fossil wood, and the impressions of vegetable stems, roots, or leaves, etc., are of the greatest importance; affording generally the most marked characters of the strata in which they occur.

These should, therefore, be particularly sought after, and their relative abundance or rarity in different situations noticed. The petrified bodies should, if possible, be kept united with portions of the rock or matrix in which they are found; and where they are numerous, in sand, clay, or any moist or friable matrix, it is in general better to retain a large portion of the whole ma.s.s, to be examined afterwards, than to attempt their separation at the time of collecting.

8. The loose materials which are found above the solid rocks, in the form of gravel, silt, rolled pebbles, etc., should be carefully distinguished from the solid strata upon which they repose. And the more ancient of these loose materials, found on the sides or summits of hills, etc., should be distinguished from the recent mud, sand, and gravel, brought down by land-floods, or rivers. The bones and teeth of animals are not unfrequently found in gravel of the former description; and the collection of these remains from distant quarters of the globe, is an object of the greatest interest to geology.

9. Besides a note of the locality, there ought, if possible, to accompany every specimen, a short notice of its geological circ.u.mstances; as:

Whether it be found in large shapeless ma.s.ses, or in strata?

If in strata, what are the thickness, inclination to the horizon, and direction with respect to the compa.s.s, of the beds? [If these cannot be measured, an estimate should always be recorded, while the objects are in view.] Are they uniform in dip and direction? curved, or contorted?

continuous, or interrupted by fissures or veins?

Is the whole cliff, or ma.s.s of strata in sight, of uniform composition?

or does it consist of different kinds of stone?

If the strata be different, what is the order in which they are placed above each other successively?

10. A label, distinctly written, should accompany every specimen, stating its native place, its relative situation, etc., etc. And these labels should be connected with the specimens immediately, on the spot where they are found. This injunction may appear to be superfluous; but so much valuable information has been lost to geology from the neglect of it, that every observer of experience will acknowledge its necessity; and it is, perhaps, in practice one of the most difficult to adhere to.

11. A sketch of a coast or cliff, however slight, frequently conveys more information respecting the disposition and relations of rocks, than the longest memorandum. If numbers, denoting the situation of the specimens collected, be marked upon such sketches, much time may be saved at the moment of collecting. But in all such cases, the memorandum should be looked over soon afterwards, and labels distinctly explaining their situation, etc., be attached to the specimens themselves.

12. The specimens should be so packed, that the surfaces may be defended from exposure to air, moisture, and friction: for which purpose, if strong paper cannot be obtained, dry moss, or straw, or leaves, may be used with advantage. Where paper is used for wrapping the specimens, they are best secured by fastening the envelope with sealing-wax.

Lastly, The collector must not be discouraged, nor be prevented from collecting, by finding that the place which he may chance to visit in a remote situation, has not a striking appearance, or the rocks within his view a very interesting character; since it frequently, and even commonly, happens, that facts and specimens, in themselves of very little importance, become valuable by subsequent comparison; so that scarcely any observation, if recorded with accuracy, will be thrown away.

The Instruments required by the geological traveller will vary, according to the acquirements and specific objects of the individual. The most essential are:

The Hammer (Sketch 5); which, for general purposes, may be of the form here represented:

The head should be of steel well tempered, about 4 inches from the face to the edge, and 1 1/4 inch square in the middle; the face flat, and square, or nearly so; the edge placed in the direction of the handle. The orifice for the insertion of the handle oval, a very little wider on the outer side than within; its diameters, about 1 inch vertically, and 0.7 across; the centre somewhat more than 1 1/2 inch from the face. The handle should be of ash, or other tough wood; not less than 16 inches long; fitting tight into the head at its insertion, without a shoulder; and increasing a little in size towards the end remote from the head, to prevent its slipping. It should be fixed in the head by means of a thin, barbed iron wedge.

For tr.i.m.m.i.n.g specimens, smaller hammers may be employed (Sketch 6): The form of the head, recommended for this purpose by Dr. MacCulloch,* is rectangular. The dimensions of the face may be 1 inch by 3/4; the height 2 1/4.

(*Footnote. On the forms of Mineralogical Hammers, Quarterly Journal Royal Inst.i.tution volume 11 1821 page 1 etc.)

It will be expedient to have always some hammers, of different sizes, in reserve.

A small miner's pick is useful for cutting out, and splitting portions of slaty rocks; or for obtaining specimens of clays, etc.

A small stone-cutter's chisel. A chisel with a handle, of the form here represented, will often save the hand of an inexpert collector, and better enable him to direct his blow.

For packing the specimens. A stock of strong paper. Sealing-wax.

Writing-paper, cut into labels. Thick gum-water, to cement the labels to the specimens.

For the Conveyance of specimens. A large bag of leather, with straps for the shoulders. Strong canvas bags, of smaller size, are very convenient for subdivision and arrangement. For the protection of crystals, or delicate petrifactions, etc., wool or cotton are necessary; and small wooden boxes (like those used for holding wafers) are sometimes required.

For distant carriage, strong wooden boxes, casks, or baskets.

The following are either essential, or useful in various degrees, for obtaining and recording observations.

Pocket Memorandum-Books, of sufficient size to admit sketches.

A Pocket Compa.s.s.

A Measuring-Tape, of fifty feet, or more.

A Telescope.

A Camera Lucida.

A Box of Colours.

The best maps should always be sought for: And, the true economy to the traveller being that which saves time, it is best to mark, or even colour the map, in the field. Notes inserted on imperfect maps, or deduced afterwards from memoranda, are less authentic; and the process is frequently neglected.

PORTABLE-BAROMETERS, with detached thermometers, are desirable; and the best instruments are ultimately the cheapest. But, unfortunately, barometers of every construction are very easily damaged or deranged. The accurate determination of heights, however, though very interesting to physical geography, is comparatively of little importance to the geologist.