188 Correspondence — Mr. S. Allport. 



fluminalis has not been found. Several species of Mollusca liave 

 been found in the Clay, but the most abundant fossil is the 

 Pentacrinus sub-basaltiformis. — Mr. Hudleston noticed that the 

 gravels exposed at the Law Courts site in the Strand were 

 much more ferruginous than those at Battersea, and. that the Clay 

 immediately underlying the gravels was altered in colour and 

 character to a much greater depth at the former than at the latter 

 section. — Mr. A. Bell thought the Cyrena fluminalis would never be 

 found in these beds at Battersea, as it belongs, he considered, to beds 

 of a different ag:e. 



ooi^i2<Es:FOi5riDE3ros. 



THE SOUECE OF VOLCANIC PEODUCTS, 



SrE, — In the last number of the Geological Magazine, the writer 

 of a notice on the re-issue of Mr. Scrope's work on Yolcanos observes, 

 that the strongest argument ih favour of a common source for all vol- 

 canic matters is the one brought forward by Mr. David Forbes, 

 " That the volcanic rocks, taken from any quarter of the world, 

 possess an absolute identity in chemical and mineralogical composi- 

 tion." 



Now, admitting the fact that basalts, trachytes, obsidian, etc., are 

 essentially the same, from whatever part of the world they may be 

 collected, and that the statement would be equally true of the older 

 rocks, I am quite at a loss to perceive in what way such facts are 

 more favourable to one rather than to the other of the two prevail- 

 ing views. 



Those views are, either, that volcanic matter is now being ejected 

 from the still fluid central portion of the globe ; or, that the molten 

 matter exists in pockets or reservoirs, at varying but still moderate 

 distances from the outer surface. On the latter supposition, the 

 molten matter would be supplied by the fusion of surrounding and 

 underlying rock-masses, or by the repeated falling-in and re-fusion 

 of previously erupted materials, none being actually forced up from 

 an imaginary central mass of fluid. 



Now, it will scarcely be denied that the rocks forming the ac- 

 cessible portions of the earth's surface are the same all the world 

 over, and have been so from the commencement of the Laurentian 

 series at least ; how much longer no one can say. Limestones, 

 sandstones, shales, and other fragmental rocks, are everywhere the 

 same, and there is no evidence to show, or reason to suppose, that 

 the materials forming the earth's crust at still greater depths differ 

 from each other in different parts of the world. It should be re- 

 membered that the elements entering into the composition of rock- 

 masses are extremely limited in number, almost universally dis- 

 tributed, and occur in the eruptive equally with the stratified series ; 

 the former may well have been derived from the fusion of the latter, 

 just as these, undoubtedly, owe their origin in many cases to the 

 abrasion and degradation of older igneous masses. 



It would appear, therefore, that a general uniformity of composi- 



