DIRK HARTOG'S ISLAND, west of Shark's Bay. A compound of rather fine-grained translucent quartzose sand, cemented by carbonate of lime, of various shades of reddish and yellowish grey. This stone has in some places the structure of a breccia; the angles of the imbedded fragments, which are from half an inch to two inches in diameter, being very distinct--but in other parts, the fracture exhibits the appearance of roundish nodules, composed of concentric shells--or bags as it were, of calcareous matter, which vary in colour, and are filled with a mixture of the same substance and quartzose sand: and the spaces between these nodules are likewise occupied by a similar compound.*
(*Footnote. The following description given by the French naturalists of the rocks at Bernier's Islands, was probably taken from a large suite of specimens; and M. Peron states (1 page 204) that it is strictly applicable to all the adjacent parts of the continent, and of the islands that were examined by the French voyagers:
Le sable du rivage (de l'ile Bernier) est quartzeux, mele d'une grande proportion de debris calcaires fortement attenues. La substance de l'ile meme se compose, dans ses couches inferieures, d'un gres calcaire coquillier, tantot blanchatre, tantot rougeatre, depose par couches horizontales, dont l'epaisseur varie de 2 a 8 decimetres (7 a 11 pouces) et qui toutes etant tres uniformes dans leur prolongement, pourroient offrir a la maconnerie des pierres de construction naturellement taillees.
Les coquilles incrustees dans ces massifs des roches sont presque toutes univalves; elles apartiennent plus particulierement au genre Natice de M. de Lamarck, et ont les plus grands rapports avec l'espece de Natice qui se trouve vivante au pied de ces rochers. Elles sont sans doute petrifiees depuis bien des siecles, car, outre qu'il est tres difficile de les retirer intactes du milieu de ces gres, tant leur adhesion avec eux est intime, on les observe encore a plus de 50 metres (150 pieds) au dessus du niveau actuel de la mer.
Quelque regularite que ces bancs puissent affecter dans leur disposition generale, ils ne sont cependant pas tous homogenes dans leur substance; il est sur-tout une variete de ces roches plus remarquable par sa structure. Ce sont des galets calcaires, agreges dans une terre sablonneuse ocracee, qui leur est tellement adherente, qu'on ne sauroit detruire cette espece de gangue sans les briser eux memes. Tous ces galets affectent la forme globlueuse, et se composent d'un grand nombre de zones concentriques, qui se developpent autour d'un noyau central d'un gres scintillant et brunatre. Ces diverses couches ont a peine quelques millimitres d'epaisseur, et affectent des nuances agreables, qui varient depuis le rouge-fonce jusqu'au jaune-clair. La disposition generale de cette breche lui donne donc quelques rapports grossiers avec le granit globuleux de l'ile de Corse; et, par ses couches rubanees, concentriques, elle a quelque chose de l'aspect des Agathes-Onyx...Les bancs de gres divers dont je viens de parler, constituent, a bien dire, la masse entiere du pays qui nous occupe, etc. (Volume 1 page 110. See also Freycinet page 187.)
The cementing limestone in the rock of this island, is very like some of the more compact portions of the stone of Guadaloupe, which contains the human skeletons, the hardness and fracture being nearly the same in both. The chief difference of these rocks seems to arise from the nature of the cemented substances; which, in the Guadaloupe stone, being themselves calcareous, are incorporated, or melted as it were, into the cement, by insensible gradation;* while the quartzose sand, in that of Dirk Hartog's Island, is strongly contrasted with the calcareous matter that surrounds it.** But, wherever the imbedded fragments in the latter consist of limestone, their union with the cement is complete.
(*Footnote. See Mr. Koenig's Paper. Philosophical Transactions volume 104 1814 page 107 etc.)
(**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 shells, in a calcareous cement; the substance of these shells has at first sight the appearance of chalcedony, and is harder than ordinary carbonate of lime.
The characters of the shells 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 shells 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 shells, 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 shelly 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 mass, 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 stalactitical concretions produced by the passage of calcareous or ferruginous solutions through sand* that they are probably of the same origin; indeed the central cavity of the stalactite 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 shell 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.)