﻿Iviii PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



and the limestone ten different elementary constituents, as the fol- 

 lowing table of the analyses of them shows : — 





Silica. 



Alumina. 



Lime. 



Magnesia. 



Protoxide of 

 Iron. 



Peroxide of 

 Iron. 



Protoxide of 

 Manganese. 



Fluorine. 



Soda. 



Potash. 



Water. 





* 





# 





* 



* 













Idocrase 



* 























Hornblende 















* 



* 









Wollastonite 









* 





* 













Epidote 



* 



* 



* 





* 















Talc 



# 





















* 





* 



* 























* 





* 







* 













It belongs to the chemist to show under what conditions these 

 elementary substances could, in the same mass, combine into- two 

 or more of those definite forms. They are phenomena of metamor- 

 phic action, of which, until explained, we cannot form any just con- 

 ception of that great operation of nature. 



But there are accompaniments of many rocks that have received 

 the name of gneiss which would seem to favour the hypothesis of a 

 hypogene rather than a sedimentary origin, viz. intercalated masses 

 of granular and crystalline limestone, and metallic ores in masses 

 and disseminated through the substance of the rock. 



MacCulloch describes* the gneiss of the island of Tiree as con- 

 taining masses of crystalline limestone, without stratification or con- 

 tinuity ; and it also occurs with a great variety of accessory crystal- 

 lized simple minerals in the gneiss of Norwayf, Sweden, Saxony, 

 Bavaria, Austria, and in different States of North America. Whence 

 the origin of this limestone thus contemporaneous with gneiss ? It 

 cannot have been derived from the detritus of any surface-stratum ; 

 for no other than an eruptive rock could as yet have formed a part 

 of the dry land. If we could suppose it to be formed by the exuviae 

 of marine organisms, it would be carrying the prevalence of animal 

 life far beyond any period in which its existence has ever been con- 

 templated, except that Sir William Logan has suggested that nodules 

 of phosphate of lime found in sedimentary rocks of Lower Cambrian 

 or even greater age may have a possible connexion with life existing 

 at that very remote period of the earth's history ; or if derived from 

 springs holding carbonate of lime in solution, that of itself would 

 be a proof of a hypogene origin. Many instances have been met 

 with of granular limestone occurring under circumstances that can 

 only be explained by supposing them to have had a subterranean 

 origin. Nearly forty years ago, Yon OeynhausenJ (elected a Foreign 



* Description of the Western Islands, vol. i. p. 48. 



t At Jaegersborg. See section by Mr. D. Forbes above referred to. 



J Noggerath's Eheinland-Westphalen, vol. i. p. 163, 1822. 



