THE MINERALOGICAL NOMENX'LATUIii: OF HOCKS. 40 



Of other minerals, iriica occurs in a series of speci(.'s like the feULspars, 

 and in rocks is found in all three Ibniis — foreign, indigenous, and second- 

 ary ; while chlorite and epidote are prohably always secondary. From 

 this it would appear that they are not suitable to designate species. 



Miner ulcxjical Nomcndfiliirc of Rocks. 



As the result of my study, I have been obliged to regard classification 

 based on mineralogy, unless it be for some varietal subdivisions, as impracti- 

 cable, because it is not a natural but an artificial method ; a system that 

 requires constant change and readaptation ; and further, one that is based 

 too much upon theory, individual judgment, and weight of authority; a 

 system that admits, even requires, the " dumping " of rocks into certain 

 places without the slightest regard to their relations of any kind, except 

 it be that they hold one or at most a few minerals in common. This rela- 

 tion is often vitiated by the observer's not taking into account whether 

 these minerals are natural crystallizations in the rock, foreign, or secondary 

 products. 



When classification is based on structure it usually separates the rock 

 into distinct species according as it is glassy, partly glassy, crystalline, or 

 porphyritic. That these distinctions are valueless, the writer thinks, fol- 

 lows from the fact that the same rock mass may show all these cases ; 

 dikes often being glassy and non-porphyritic on the edges, and crystalline 

 and porphyritic towards the middle. The granitic structure indicates pro- 

 bably a certain depth at the time of crystallization, but that this may 

 practically be slight has been shown by the lava flows of Keweenaw Point, 

 some of which are fine-grained on the surface, and coarsely crystalline 

 (granitic or diabasic) towards the base. 



Section VI. — Naming Rocks according to the Geological Age. 



This question has been so well discussed by All])ort,* Dana t and others, 

 that but little needs be said upon the subject here. Chemical, microscopi- 

 cal, and geological evidence all point to the fact that this division is not a 

 natural one ; and so far as my work has gone, the original characters of the 



* Geological Magazine, 1S71 (1). viii. 249; 1S75 (^), ii. oS3; Quart. Jour. Geol. Sue, lS:i, xxx. 

 529. 



t Amer. Jour. Sci., 1S78 (3), xvi. 336. 



