92 GEOLOGY OF THE HIGH PLATEAUS. 



therefore, form very slowly, and time becomes an important element in 

 determining the whole amount of crystallization. It is easy to see that an 

 eruptive lava, rapidly cooling under the sky, may remain but a short time 

 at the temperatures at which crystals can form. On the other hand, an 

 injected or plutonic mass may long retain its high temperature. In the 

 fonner case the rock finally becomes half-crystalline, in the latter case 

 wholly crystalline. That this is the explanation of the textural differentia- 

 tion of the plutonic and erupted rocks seems very probable, and thus tex- 

 ture becomes associated with the genesis of the rock and the causes which 

 have made it what it is. 



There is a very respectable school of German lithologists who make the 

 geological age of igneous rocks a primary criterion of classification. They 

 place all igneous rocks, whose intrusion or eruption occurred prior to Ter- 

 tiary time, among the granitoid or porphyroid classes, and all Tertiary or 

 Quaternary eruptives among the true volcanics. For example, all augitic 

 plagioclase rocks of Pre-Tertiary origin are regarded as diabases, mela- 

 phyres, or augitic porphyries, &c., while all of Post-Cretaceous origin are 

 regarded as basalts, " trachydolerites," &c. Such a classification most as- 

 suredly could be defended only upon the assumption or ascertained fact 

 that certain characters are found in the more ancient eruptives which are 

 wanting in the more recent ones and vice versa. Is this assumption uni- 

 versally true 1 I hold that it is not. That in a great majority of cases the 

 Pre-Tertiary igneous, as we now see them, are granitoid or porphyroid, 

 while those of later epochs are volcanic, thus presenting textural differences, 

 is undeniable. But exceptions exist, and they are highly important ones. 

 It is possible, not to say probable, that many more exceptions might be 

 looked for than can at present be specifically named if there were not a 

 certain looseness in the use of names, by which rocks of the volcanic tex- 

 ture are classified with the granitic groups. This is especiall}'^ observable in 

 the augitic divisions. The augitic rocks of the Palaeozoic system, notably 

 those of Carboniferous age, are frequently classed as diabase, when more 

 properly they might be in many instances placed among the dolerites or 

 basalts. Indeed, some intelligent observers, wlio are not committed in any 

 way to the foregoing generalization, do not scruple to call the intruded and 



