Volume II Part 60 (1/2)

(**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?