Geology and Mineralogy. 481 



the Alps and in the central Highlands of Scotland, to mention 

 only two localities, will be admitted by all who are familiar with 

 those regions. Again, if the associated rocks possess the composi- 

 tion of igneous products, it seems equally reasonable to conclude 

 that they are of igneous origin. Such a series we find in the 

 northwest of Scotland, in the Malvern Hills, and at the Lizard. 

 In applying the test of chemical composition, it is very necessary 

 to remember that it must be based not on a comparison of indi- 

 vidual specimens, but of groups of specimens. A granite and an 

 arkose, a granitic gneiss and agneiss formed by the metamorphosis 

 of a grit, may agree in chemical and even in mineralogical composi- 

 tion. The chemical test would therefore utterly fail if employed 

 for the purpose of discriminating between these rocks. But when 

 we introduce the principle of paragenesis it enables us in many 

 cases to distinguish between them. The granitic gneiss will be 

 associated with rocks having the composition of diorites, gabbros, 

 and peridotites; the sedimentary gneiss with rocks answering to 

 sandstones, shales, and limestones. Apply this test to the gneisses 

 of Scotland, and I believe it will be found in many cases to 

 furnish a solution of the problem." 



" ... The origin of gneisses and schists, in my opinion, is to 

 be sought for in a combination of the thermal and dynamic agen- 

 cies which may be reasonably supposed to operate in the deeper 

 zones of the earth's crust. If this view be correct, it is not im- 

 probable that we may have crystalline schists and gneisses of 

 post-Silurian age in the northwest of Europe formed during the 

 Caledonian folding, others in central Europe of post-Devonian 

 age due to the Hercynian folding, and yet others in southern 

 Europe of post-Cretaceous age produced in connection with the 

 Alpine folding. But if the existence of such schists should ulti- 

 mately be established, it will still probably remain true that rocks 

 of this character are in most cases of pre-Cambrian age. May 

 not this be due to the fact, suggested by a consideration of the 

 biological evidence, that the time covered by our fossiliferous 

 records is but a small fraction of that during which the present 

 physical conditions have remained practically constant ?" — Nat. 

 Science, Oct. 1893. 



2. Ore Deposits of the United States; by Prof. J. F. Kemp. 

 8vo, pp. 302. New York, 1893 (The Scientific Pub. Co.).— The 

 author after several preliminary chapters treating on classifica- 

 tion, the mode and occurrence of ores as deposits and in mineral 

 veins, then proceeds to the main body of the work. This takes 

 up each of the metals in turn and describes concisely one occur- 

 rence typical of the method of classification. The similar occur- 

 rences in other localities are mentioned and a full list of the 

 literature is given. Thus all the various methods of occurrence 

 of each metal and its ores are shown. The volume is copiously 

 illustrated with cuts and diagrams. The author has collected 

 within brief compass a vast amount of useful information while 

 the very full bibliography greatly enhances its value. The appear- 



