40 PROCEEDINGS OF TIIE GEOLOGICAL SOCIETY. [N*QV. 19, 



General Portlock, also, states* that in Londonderry coarse arenaceous 

 schists, containing Orthis grandis, &c, rest directly upon, and skirt 

 granite and hornblende-rock (see section, p. 230). Mr. Jukes 

 observed a similar fact among the Cambrians of "Wicklow f (see 

 section) ; and Professor Ramsay informs me that he has seen it in 

 {Scotland. 



b. Singes in the Cambrian Epoch. — The Cambrian epoch then is 

 marked by three distinct conditions, which perhaps represent stages 

 caused by changes in the relative level of land and sea ; and hence 

 the want of constancy in its constituents. 



These conditions are : — 



1. The under rock, Laurentian or Huronian, having been ex- 

 posed above water, no deposit whatever took place until the Primor- 

 dial epoch of the Silurian period arrived ; in some places with 

 abundance of life. 



2. The parent rock having been submerged in shallow and dis- 

 turbed waters, native conglomerates were produced by well-known 

 processes (Scotland, North America, &c). 



3. These parent rocks having subsided into great depths, arena- 

 ceous, argillaceous, and other deposits took place, sometimes to the 

 thickness of 25,000 feet and more ; such deposits becoming after- 

 wards more or less metamorphosed. 



c. Strati graphical characters, — Passing by § 4, we find in reference 

 to § 5, on examining the Synoptical Table, that the succession of beds 

 in this group of strata (schists, grits, conglomerates, &c.) is quite 

 irregular. This arises from local causes ; and the want of stratigra- 

 phical agreement, it is remarkable to notice, is as strong in contiguous 

 districts as in those more remote. 



In considering § 6, which treats of the thickness of the Cambrian 

 strata, I believe that unusually great accumulations have no great 

 geological importance. I deduce from them that a change of level 

 has occasioned heavy denudations somewhere, followed, of course, by 

 loadcd currents, which have been suddenly arrested at the place of 

 accumulation. One effect is, that this thick deposit has few or no 

 fossils, for more than one obvious reason. We see this exemplified 

 by the ten or twelve thousand feet of Devonian Sandstone (without 

 organic remains) in Kerry and Cork, Ireland % ; and in the three 

 thousand feet of the same rock, forming the Catskill Mountains, 

 JNew York, containing few animal remains, but many plants. The 

 thickening of the middle Carboniferous Limestone in Derbyshire and 

 Yorkshire is another similar instance. "We must not, therefore, ex- 

 aggerate the importance of thickening in the Cambrian. 



d. Organic elements. — As regards § 7, the absence from this set of 

 beds of phosphorus and such elements is generally believed, but upon 

 what authority I know not. Instead of numerous analyses in sup- 

 port of this opinion, I only recollect a very few by Mr. D. Forbes and 

 by Dr. Lyon Playfair. But we must not forget Professor Daubeny's 



* Survey of Londonderry, p. 303. 



t Jukes and Wylie, Journ. Dublin Geol. Soc. vol. vi. 



\ Jukes, Student's Manual, p. 409. 



