97 



due to derivation, and is no indication of synchronism, the petro- 

 silex being regarded as a recomposed rock formed from the 

 debris of the granite. There are instances where this might very 

 well be the case, but in general the facts lend little support to this 

 view. Besides, such an hypothesis would make the petrosilex a 

 shallow water deposit, since it everywhere lies in close proximity 

 to or directly upon the granite masses from which, according to 

 that view, it was derived. Whereas, reasoning from analogy, 

 it is far more probable that this rock, which is of very uniform 

 composition and almost chemical fineness, and which is to a 

 large extent characterized by a banded or stratified structure 

 of remarkable evenness and regularity, was formed amid con- 

 ditions of long-continued tranquillity such as obtain only in the 

 greater depths of the ocean. Possibly we have something 

 analogous at the present day in the deposits of aluminous 

 silicate (red clay), which recent explorations show to be form- 

 ing over millions of square miles in the abysses of the ocean, 

 and which appear to take their origin chiefly in the volcanic dust 

 and pumice disseminated by winds and currents, these materials 

 being usually more or less extensively decomposed subsequently 

 to their deposition ; and I would suggest in this connection that 

 the silicious shells of Diatoms and Radiolaria which have been 

 rained down upon the ocean floor in a steady shower uninterrupted 

 through countless ages are a possible source of the free quartz 

 which, in addition to alkalies, would be needed to give this alumi- 

 nous deposit the chemical aspect of a petrosilex. 1 Assuming, then, 

 that the transition in question, if it really exists, is due to 

 alteration and indicative of at least approximate synchronism, 

 and that it is a transition from the petrosilex to the granite, 

 we may premise, in the first place, that, on a priori grounds, 

 such a gradual passage is in the highest degree probable. All 

 authorities are agreed that, chemically and mineralogically, these 

 rocks are essentially identical, differing only in texture ; and in 

 this region, as, doubtless, in others, the range of variation in the 



i For further remarks under this head, see my paper " On a Possible Origin of 

 Petrosilicious Rocks," Proo. Boston Soc. Nat. Hist., xx. 



OCCAS. PAPERS B. S. N. H. —III. 7 



