506 Clarke — Geology of Western Australia. 



general mass ; and, perhaps, subsequent researches will justify the 

 suggestion, that the occurrence of granite over so wide an area in 

 Western Australia, is due to a boss-like^ elevation, certainly after the 

 period of the older Palgeozoic rocks, the traces of which, though 

 limited in extent, are widely scattered and sufficiently repeated to 

 sustain the conclusion, that they once existed probably over the 

 whole granitic area; and that from the calcareous coating of the 

 granite at King George's Sound and Recherche Archipelago some of 

 it may be even of Post-tertiary elevation. 



' The dome-like or boss-like form of granite is a feature of frequent occurrence. It is 

 of concretionary origin, and distinguishes especially those hornblendic granites which 

 are connected with auriferous deposits. But it occurs also in other granitic regions, 

 as in the mountains of Central Asia. Where the great bosses meet there is generally 

 a depression, which if repeated in a giyen direction, may give the idea of a fissure ; 

 this may also be the case where joints traverse the rock. Mr. Lefroy mentions intu- 

 mescences of granite, a term well describing the nodular or concretionary structure. 

 He also mentions that near the head of di-ainage to the westward, there is a dip of 

 about 2 feet in the mile to the south-west. Eyre also mentions that nearer the coast 

 the granite has a slope to the south-westward. A slope of 2 feet in a mile could 

 scarcely occur except on the summit of a nearly level mass. That the granite in the 

 region under discussion must have, occasionally at least, a true nodular structure, may 

 be seen on the nearest part of the coast to Mr. Hunt's furthest, viz : about Es^jcrance 

 Bay, where the granite exfoliates in decomposition, peeling off in concentric layers 

 like those of concretionary trap. Mr. Lefroy mentions similar featui-es in 32-0 S. and 

 118° 19' E. at Burra Kukkin. This particular species of granite is about Esperance 

 Bay full of garnets, and garnetiferous granite is well known to be not of the most 

 ancient class, and, besides the inferences from this fact, that the rocks must have con- 

 siderable slopes is to be inferred from the fact that the cliffs of the Australian Bight 

 have, in some places, a vertical thickness of 600 feet, resting on the edges of the granitic 

 base, which begins to crop out on the west side of the great arc of the coast at about 

 124° E., and on the east side near Fowler's Bay about 132° E., giving to the hollow 

 between the granitic intumescences a breadth of 500 miles. We may, therefore, 

 safely assume that such a hollow is formed by the slopes of the great dome-like masses 

 composing the granite base. That towards the ocean this concretionary or nodular 

 structure may furnish slopes of enormous extent, may be gleaned from a fact stated to 

 me by my friend the late Captain Owen Stanley, E.N., F.E.S., who in H. M. S. 

 Rattlesnake obtained soundings when fairly inside the horns of the Bight, at a depth 

 of fou7- miles. That this is far from surprising may be considered by reference to the 

 data given for the mass of strata supposed to be denuded, in the preceding remarks. 

 For with a slope of the granite or other rock not greater than one degree in the mile, a 

 depth of more than four miles would be reached along the versed sine of the Bight at 

 a distance from the cUffs of 234 miles, which is within the chord along the 36th 

 parallel. Deep as is this depi-ession of the sea bottom, it is quite evident that the 

 Tertiary and underlying deposits may obtain their present position without any extra- 

 ordinary concurrence of circumstances. The great width of the Bay and the depth 

 indicated for the ocean off the cliffs of Bundah, justify the probable opening of the 

 Strait aUuded to above. Nor can it be without interest to recollect that Flinders, on 

 his survey of the Bight, stated his belief that the sea would be found behind them 

 (vol. i. p. 97). He distinctly refers to a gradual subsiding of the sea, or sudden con- 

 vulsion of nature, which, however, can have no bearing on the real facts of the case, 

 and to the cylindrical sandy concretions of Bald Head, which have another and truer ex- 

 planation tlaan he supposed, and'yet which, if interpreted in Ms way, would prove eleva- 

 tion of the coast, which is so far correct. At from 8 to 15 miles from the shore at the 

 head of the Bight, the depth according to D'Entrecasteux and Flinders is from 27 to 

 30 fathoms English ; the depression, therefore, observed by Stanley is much further 

 out, and beyond the base of the Tertiary platform which extends from the head of the 

 Bight on the south-west side to a parallel with the chord of the Bight about 140 

 miles south. There is room and depth enough to allow for other formations as well 

 as the Tertiary, which, for want of researches, cannot, however, be proved. — W. B. C. 



[To he concluded in our next number.^ 



