38 



STRATIFIED ROCKS. 



S. Spot ' the returnin g *» water (not being saturated) would 



dissolve these saline crystals, leaving cubical cavities in the 



7all-case41. 



mud which contained them. The tide would bring with it 

 a fresh deposit of fine sand, a portion of which would pour 

 into the cavities formed by the crystals, and the remainder 

 would form a homogeneous stratum immediately above. 

 ("On Pseudomorphous Crystals of Chloride of Sodium in 

 Keuper Sandstone," H. E. Strickland, Esq., F. R. S. 

 F.G.S. Journal of Geological Society, Dec. 1, 1852.) 



Wall-case 41. 



These Specimens from Nos. 1 to 48 arc all varieties of 

 Conglomerates. They may be divided into two chief varie- 

 ties, viz,, those in which the embedded fragments are an- 

 gular, constituting brecciated conglomerates ; and those in 

 which the embedded fragments are rounded, forming the 

 most numerous class of ordinary conglomerates. In the 

 brecciated conglomerates, the specimens have not been sub- 

 jected to much attrition, but have been dropped in a more 

 or less angular state amid the finer sediment that forms 

 the cementing base. — (Nos. 2, 5, 6, 16.) 



n the ordinary conglomerates the fragments have been 

 rounded by attrition, by the action of breakers, &c, like the 

 pebbles of an ordinary beach. In different specimens the 

 cementing matter is of different kinds ; in some the stones 

 are cemented by lime, in others by oxide of iron, in others 

 by both, in others the agglutination has perhaps been aided 

 by the decomposition of felspathic matter, and the forma- 

 tion of new silicates of soda or potash ; in others which 

 seem purely siliceous it would appear as if small quantities 

 of silica had been dissolved and re-deposited among the 

 quartz grains and pebbles, and in many cases pressure has 

 had a great deal to do with the consolidation of the rock. 



Conglomerates are of all degrees of coarseness ; in the Old 

 Red Sandstone of Scotland, and the Lower Red or Permian 





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