496 Trof. T. Sterry Hunt — Elements of Primary Geology. 



8. If we restrict the term eruptive to such rocks as have 

 evidently come in a fused or plastic condition into their position 

 among and above previously formed masses, we may on miner- 

 alogical grounds divide them into two classes. 



I. Eocks consisting essentially of basic silicates, including much 

 alumina, lime, magnesia, and ferrous oxyd, of which dolerite is 

 the type. These we regard as portions of the primitive plutonic 

 mass, which however, as already explained (§ 2), has, through 

 successive ages, become more or less modified alike by additions 

 and subtractions through the crenitic process, and by crystallization 

 and partial eliquation, thereby giving rise to the considerable 

 variations in composition met with in these basic rocks in different 

 areas and in different periods. The stratiform arrangement of the 

 elements of these, due to movements of flow, occasionally met with 

 in such masses, simulates, as is well known, the structure of 

 stratified crenitic masses. This has led some theorists to assign 

 to these plutonic rocks an aqueous and so-called metamorphic 

 origin ; while the same resemblance has been, alike by endoplu- 

 tonists and exoplutonists, made an argument for maintaining 

 a plutonic origin for all stratiform crystalline rocks. 



II. Besides the basic eruptive rocks, which are to be regarded 

 as truly plutonic, are those mineralogically very unlike masses 

 which resemble in composition the predominant type of the earlier 

 crenitic rocks, and are often confounded with them. Such are the 

 trachytes and the clearly eruptive granites, which, it is maintained, 

 are but softened and displaced portions of older crenitic masses, 

 and may be designated pseudojjlutonic rocks. 



9. Besides these masses of once softened and plastic material 

 — whether plutonic, or pseudoplutonic and of crenitic origin — 

 which have been erupted after the manner of lavas, there are two 

 other kinds of rock-masses, which, from their geognostic relations, 

 are frequently regarded as eruptive. Of these the first is the 

 veinstones or endogenous rocks — alike quartzose, calcareous and 

 granitic — already mentioned (§1), themselves of crenitic origin, 

 and formed, although in fissures or cavities, under chemical con- 

 ditions not unlike those which gave rise to the greater masses of 

 non-plutonic primary rocks. As regards the second kind of masses 

 above referred to, it often happens that disrupted portions of crenitic 

 rocks have, without softening or change of state, been forced, as the 

 result of local movements, among softer and more yielding strata. 

 Such intrusions of rigid amongst plastic masses have in many cases 

 caused the former to be regarded as rocks erupted after the manner 

 of lavas. 



10. In the case of crystalline or colloidal rocks evidently posterior 

 to the enclosing material, the question will therefore arise whether 

 a given mass may be (1) a truly plutonic rock ; (2) a crenitic deposit 

 in situ, as a granitic veinstone ; (3) a crenitic mass from a lower 

 horizon, which by softening and displacement has assumed a pseudo- 

 plutonic form ; or (4) a portion of rock, either crenitic or plutonic 

 in origin, which, without having itself been softened, has been forced 

 among soft and yielding materials, displacing these. 



