86 ERNEST H. L. SCHWARZ 



the hornblende, seems rather to indicate that the blocks came up 

 in the lava from below. 



I cannot forbear in this connection from drawing attention to 

 a similar result which was obtained in the earliest experimental 

 work which was carried out on the melting-points of rocks. Sir 

 J. Hall, in heating some blocks of basalt from Arthur's Seat, near 

 Edinburgh, found that at ioo° of Wedgwood's pyrometer the whole 

 was changed to a pure black glass, but at 60° the felspar remained 

 unchanged while the hornblende disappeared, and formed a glass 

 along with the basis of the stone.' 



The further action of heat in volcanoes is shown in the Night- 

 ingale rocks, where fragments have been included in the liquid 

 magma and in part remelted. And a still further stage, possibly, 

 is exhibited in the rocks of Tristan d'Acunha, where sphene occurs 

 in great abundance.^ This mineral, together with perovskite, 

 occurs in certain eruptive rocks, which, as far as I have observed 

 them personally, suggest that sedimentary rocks have been ab- 

 sorbed in the molten magma. Perovskite is an accessory mineral 

 in the melilite basalts of the Karroo volcanic pipes, and also in the 

 mass of melilite basalt which occurs in the faulted Cretaceous rocks 

 of the south coast of the Cape Colony ;3 it can be regarded, in fact, 

 as an alteration product of the ilmenite in the original magma, 

 from being brought into contact with hmestone in the zone of intense 

 pressure. The extraordinary abundance of sphene in the Tristan 

 d'Acunha rocks I am inclined to view as the result of the inter- 

 action of a rock magma containing titanium acid and a limestone. 



The final stage of the temperature gradient is that in which 

 the rock is entirely molten, and the molecules of the various elements 

 left to sort themselves into groups to form minerals without any 

 influence from the inclusion of extraneous solid materials. If we 

 could know the whole history of igneous rocks, perhaps these too 

 might tell us something about the substructure of the earth beneath 

 the volcanic vents, for it is not an entirely untenable theory that 



1 "Experiments on Whinstone and Lava," Transactions of the Royal Society (Edin- 

 burgh, 1798). 



2 Transactions of the South African Philosophical Society, Vol. XVI, p. 46. 



3 Annual Report of the Geological Commission, 1898, p. 62; ibid., 1903, pp. 43-67. 



