THEORIES OF ALTEUATIOX DISCI'SSED. 17 



advocate a sort of perpetual motion. Accordinf^ to lliem these rocks are 

 born, grow old, and die, and tlieir remains are raised again and again, that 

 the process may be repeated. The writer accepts the birth, old ago, decay, 

 and death ; but he doubts the resurrection and believes that such views 

 are opposed to physical laws. 



A crystalline structure is indigenous in any eruptive rock, if it remains 

 in a condition that allows it to slowly crystallize ; and this structure is not 

 therefore any proof of great age in a rock, or a sign that it was formed at 

 great depth.* 



From the above it would follow that such rocks as the felsites cannot be 

 taken as characteristic of certain ages (Arvonian or Iluronian) ; but if — as 

 the writer, with others, holds — they are old rhyolites, they have been formed 

 in all ages. Again, while they may have been deeply covered with detrital 

 beds, there is no necessity for such a burial, or any proof that they were 

 once thus covered, any more than there is that the modern rhyolites have 

 been. 



Also, the claim for long times for the formation of rocks which are fine- 

 grained and fossiliferous cannot always be allowed ; as for instance, the 

 Florissant shales,! or a large deposit of fine, dust-like powder, observed in 

 the vicinity of the Black Hills by my colleague, Mr. Samuel Garman. This 

 powder is made up of minute fragments of volcanic glass, forming a bed 

 several feet in thickness. If it had not been for the revelations of the 

 microscope, would not some geologist be computing the number of thou- 

 sands of years it would take to form these deposits " as Nile mud," when 

 perhaps, a few weeks or even days were sufficient for this purpose. If 

 the deposits in question had been subjected to sufficient alteration to 

 obliterate the original texture, who would have been able to prove the 

 falsity of the theory of a slow deposition of the material as an ordinary 

 sediment ? 



Another illustration is afforded by the Lake Superior sandstone, which 

 shows that extreme care is required to ascertain the conditions under wlii(^h 

 any deposit formed, before the length of time required for its formation shall 

 be estimated. $ 



To the objections offered to lavas being the same from all time, on 

 account of the difficulty of believing that the same portions of the earth's 



* Bull. Mus. Conip. Zool., 1S80, vii. 111. 



t Bull. U. S. Geol. Survey, 1881, vi. 286, 287. 



+ Bull. Mus. Comp. Zool," ISSO, vii. 177, 118. 



