26 KE VIEWS. 



The granite of Cornwall is of carboniferous date in all probability ; the 

 granite of Leiuster is post-silurian, although ante-carboniferous ; the gra- 

 nite of Down is post-carboniferous ; and that of Donegal is, probably, 

 also carboniferous, or only somewhat antecedent in age. Where, then, 

 are the presilurian Granites which supplied the materials of the thick masses 

 of slate we have mentioned ? 



Ordinary Granites consist of about 25 per cent, of Quartz, 60 per cent, 

 of Felspar, and 15 per cent, of Mica. When such a rock is exposed to 

 the action of water and converted into a stratified rock, its felspar loses 

 its alkalies, while the mica remains nearly unaltered, and the resulting 

 rock will be a compound of Impure Kaolin, Quartz, and Mica, mixed in 

 proportions not absolutely fixed, but, probably, never very far from the 

 following: Quartz 25 per cent., Kaolin 18 per cent., and Mica 57 percent. 



Another circumstance which characterises the early Silurian slates is the 

 small proportion of lime and magnesia found in them ; this falls in well with 

 the supposition of their being derived altogether from Granitic Rocks. It is 

 certainly a curious fact that the quantity of lime in the crust of the earth 

 increased with the increase of organic life, and, also, probably, the quantity 

 of soda and phosphorus — as if some change of mineral conditions of the 

 surface of the globe had rendered it more fit for the habitation of or- 

 ganised beings. 



We must, however, return from this digression to Prof. Ansted's book. 

 One of the most interesting parts of the fifteenth chapter is the description of 

 the Oolitic coal fields, including the remarkable beds of India, pp. 375-380. 

 From the specimens of fossil plants recently sent from the Rajmahal Hills, 

 there can be little doubt of their being more closely allied to the Oolitic 

 coal plants of Scarborough than to the vegetation of the true carboniferous 

 epoch. The Palaeontological portion of the book is respectable ; the text 

 written to suit the woodcuts, as before ; but there is nothing in it which 

 calls for special praise or mention. 



PRACTICAL GEOLOGY. 



We regard this portion of the book as the best written and most useful 

 Manual of Mining with which we are acquainted, in the English language; 

 and we think that it supplies a valuable aid to the student of Mining Engi- 

 neering. It is fairly entitled to be considered as a new addition to the 

 former edition of this book, as it is doubled in extent, consisting of one 

 hundred and twenty-three pages, instead of sixty-one, and the whole is 

 carefully arranged for the use of students. S. H. 



