CLASSIFICATION IX JJTIIOLOfJY. — GENERAL CON'CLL'.SIONS. 07 



111 practically employing tlie principles given before, all kinds of rock.s, 

 whether from the earth or heavens, are natural! v to he (le.seribed an<l 

 arranged, but only a limited portion of this work can be d(;ne here. In 

 the execution of this the most l^asic rocks kncnvn will be taken, ami the 

 gradations between the different states traced as far as practicable. 



The order will be to pass from the glassy to the most perfectly crystalline 

 state ; from the least altered towards the most altered ; from the most ba.^ic 

 towards the most acidic; from the non-fragmental to the fragmental or 

 clastic. In the case of the fragmental rocks, it is intended to proceed from 

 the least altered to the most altered ; that is, from those little consolidated 

 towards those most indurated ; from the unaltered towards those most highly 

 metamorphosed. In the wide range of rocks studied, it has been found as a 

 rule that the macroscopic and microscopic characters could generally be 

 inferred one from the other; and that, likewise, from these the chemical com- 

 position could be declared, within certain general limits. Indeed, by the 

 method of classification pursued here, a bit of vnaUered groundmass, not pure 

 glass, sufficient to fill the field of a No. 7 or 9 Ilartnack objective, is enough 

 to enable one to tell the species to which the rock belongs, and to give some 

 general idea of its composition, both chemically and mineralogically. 



In applying the earlier given principles of nomenclature and classifica- 

 tion, the following method has been practically adopted here : All the 

 rocks, from one end of the series to the other, are divided into species or 

 groups; and each of these species possesses the general characters that have 

 usually found voice in the generally established nomenclature. Since the 

 most typical forms are those of the modern eruptive rocks, their names, 

 when practicable, have been selected, and in them the limits of the species is 

 made as nearly as possible coincident with those made by the leading lithol- 

 ogists. Of course they are not absolutely identical, for natural boundaries 

 do not correspond entirely with the artificial fences made l)y man. Tender 

 these specific or group names — such as basalt, andesite, trachyte, and 

 rhyolite — are placed, as varieties, the altered, as well as older (pre-Ter- 

 tiary) forms; which it is thought were once identical with the unaltered 

 or modern forms. The variety names are also used as nearly as possible in 

 their more general applications; but it is often fi)und that the artificial boun- 

 daries of these varieties (regarded by other lithologisis as distinct species) 

 carry them into two or more of the species, as that term is used here; <•.//., 

 melaphyr is, as a rule, a name given to the fine-grained older and altered 



