Geology and Mineralogy. 475 



depths of fusion along with the igneous rock, and also, the agency 

 of any vaporized materials derived from the walls of the fissures 

 through the heat, any part of which walls may, as far as is known, 

 yield ores, and the last few miles of which may intersect rocks 

 containing a per cent or so of moisture (06 per cent would be a 

 pint to the cubic foot of rock), or holding bodies of fresh or salt 

 water between the beds. Subterranean waters have come from 

 above and may be included among "external agencies;" and if 

 so, the theory is wide enough to cover a large part of vein-making, 

 and so far falls in with views that have been advocated by others. 



4. On Slaty Cleavage and allied rock structures toith special 

 reference to the mechanical theories of their origin ; by Alfred 

 Habkee, M.A., F.G.S. Brit. Assoc' Rep. for 1885.— This is a 

 thorough, partly mathematical, and judicious discussion of the 

 subject of slaty cleavage. The general conclusion is expressed 

 in the following statement from page 38 : " The theories discussed 

 in the foregoing pages make the cleavage of rocks a result of 

 lateral thrust operating throughout larger or smaller tracts of 

 country; and the extreme stages of the structure, which involve 

 mineralogical as well as lithological changes, a consequence of 

 the intense stresses in the earth's crust to which mountain-systems 

 owe their structure." While admitting that the impression of 

 the cleavage structure is of later date, in the main, than the tilt- 

 ing and flexures observable in them, he still regards the cleavage 

 and folding as concomitant though not simultaneous effects of the 

 same lateral pressure. 



5. Index der Krystallformen der Mineralien von Dr. Victoe 

 Goldschmidt. Vol. I, part 1. 280 pp., large 8vo. Berlin, 1886 

 (Julius Springer). — The task which the author has undertaken is 

 one of very great magnitude, namely, the presentation of a com- 

 plete catalogue of the forms observed on the crystals of each 

 mineral species, with the names of the observers, the letters em- 

 ployed by different authors, the literature, and so on. The part 

 now issued, of 288 pages, is only half of the first volume, and the 

 whole work is to consist of six such parts in three volumes. The 

 Introduction includes the development of the special symbols 

 employed by the author, with the forms of calculation adapted to 

 them, and also a very thorough explanation of the mutual rela- 

 tions of the different systems of notation for the planes of crystals 

 from Hatiy down. The author's general symbol (except in the 

 hexagonal system) takes the form pq, which is equivalent to the 

 hkl of Miller reduced to a form where the I is unity, that is, in 



h k 

 general pq= ~r~T' ^ * s snown ^ nat ^' 1S f° rm brings out clearly 



the relations of the planes and lends itself easily to purposes of 

 calculation. Those who have become familiar with the system to 

 which the name of Miller is ordinarily attached will be slow to 

 believe that any other system can be at once so clear and con- 

 venient, and will be likely to go further and question the desira- 

 bility of introducing a new system at a time when so great a 



