350 Notices respecting New Booh :— Darwin on the 



has been deposited on the underside of ledges of rocks or in fissures, 

 it appears always to be of a pale pearly-gray colour, whereas, in other 

 situations, it has a dark gray or even a jet-black colour. This in- 

 duces the author to suppose 



" that an abundance of Hght is necessary to the development of the dark 

 colour, in the same manner as seems to be the case with the upper and ex- 

 posed surfaces of the shells of living mollusca, which are always dark, com- 

 pared with their under surfaces and with the parts habitually covered by 

 the mantle of the animal. In this circumstance, — in the immediate loss of 

 colour and in the odour emitted under the blow-pipe, — in the degree of 

 hardness and translucency of the edges, — and in the beautiful polish of the 

 surface, rivalling when in a fresh state that of the finest Oliva, there is a 

 striking analogy between this inorganic incrustation and the shells of living 

 molluscous animals*. This appears to me to be an interesting physiolo- 

 gical fact." 



In continuation of the geological history of Ascension are described 

 some singular laminated beds, alternating v^'ith and passing into ob- 

 sidian. These consist, — 1, of a pale gray, irregularly and coarsely la- 

 minated harsh -feeling rock, resembling clay-slate which has been in 

 contact with a trap-dike, and easily fusing, like the following varieties, 

 into a pale glass ; 2, a bluish-gray or pale brown, compact, heavy, 

 apparently homogeneous stone, in which, however, under a lens, a 

 crystalline fracture and even separate minerals can be observed ; 3, 

 a stone resembling the last, but streaked with crystalline white hairs 

 of quartz and green transparent needles of diopside ; 4, a compact 

 crystalline rock banded with innumerable layers seeming to be com- 

 posed chiefly of felspar, and containing numerous perfect crystals of 

 glassy felspar placed lengthways, minute black specks of hornblende 

 or augite, and red specks of oxide of iron ; 5, a compact heavy rock, 

 not laminated, with an irregular, angular, highly crystalline fracture, 

 abounding with distinct crystals of glassy felspar, and having the 

 crystalline felspathic base mottled with a black mineral, and the en- 

 tire rock strikingly resembling, in aspect, a primitive greenstone. 



We have been thus particular in describing after our author the 

 characters of these rocks, because the knowledge of those characters 

 is essential to the accurate comprehension of the history of their pas- 

 sage into obsidian, which, with its accompaniments, we deem one of 

 the most instructive and suggestive portions of the work, and which, 

 therefore, we will extract entire. 



The five varieties of rock just described, 



" with many intermediate ones, pass and repass into each other. As the 

 compact varieties are quite subordinate to the others, the whole may be 

 considered as laminated or striped. The laminae, to sum up their character- 

 istics, are either quite straight, or slightly tortuous, or convoluted ; they are 

 all parallel to each other, and to the intercalating strata of obsidian ; they 



* "In the section descriptive of St. Paul's Rocks, I have described a glossy, 

 pearly substance which coats the rocks, and an allied stalactitical incrus- 

 tation from Ascension, the crust of which resembles the enamel of teeth, 

 but is hard enough to scratch plate glass. Both these substances contain 

 animal matter, and seem to have been derived from water infiltering through 

 birds' dung." 



