202 



UNSTRATIFTED OR IGNEOUS ROCKS. 



axes and peaks of nearly all great mountain-ranges ; lb', in vertical or 

 nearly vertical sheets, filling great fissures in stratified or in other 

 igneous rocks ; c, in extensive horizontal sheets overlying the stratified 

 country rock, as if outpoured on the surface ; c\ lying conformably 

 between strata, as if forced in a melted condition between them, or else 



0^S^yr i~C-~ *■* x < v limiil Eruptives. 

 ^ K>;;<-| Granitics. 



|~^1 Metamorphic. 



Palaeozoic. 

 |:->;tj Mesozoic. 

 |=^z ]Cenozoic. 



Fig. 180.— Diagram showing Mode of Occurrence of Igneous Rocks. 



outpoured on the bed of the sea and afterward covered with sediment ; 

 and, d, in tortuous veins connected with the great underlying masses. 

 All these positions are illustrated in Fig. 180. In all these modes of 

 occurrence the observed rock is connected with an underlying mass, 

 of which it is but an extension. 



Extent oil the Surface. — The appearance of these rocks on the sur- 

 face is far less extensive than that of the stratified rocks. Certainly 

 not more than one tenth of the land-surface is composed of them. 

 But, beneath, they are supposed to constitute the great mass of the 

 earth. 



Classification of Igneous Rocks. — Igneous rocks are best classified, 

 not by means of their relative ages, but partly by their mineralogical 

 character and partly by their mode of occurrence. By this method 

 they most naturally fall into two primary groups — viz., the Plutonic 

 or massive, and the volcanic, or true eruptive rocks. The rocks of the 

 first group occur in great masses ; those of the second group injected 

 into fissures or outpoured on the surface. The former are entirely 

 crystalline (holo-crystalline), and usually very coarse-grained (macro- 

 crystalline) ; the latter are usually finer grained (micro-crystalline), or 

 imperfectly crystalline (crypto-crystalline), or partly or even wholly 

 glassy. The former seem to have solidified in, situ (indigenous) ; the 

 latter have been evidently displaced form their original position (exot- 

 ic). The two groups, however, pass by insensible gradations into each 

 other, so that the distinction is more or less artificial, and the same 

 rock may sometimes be found in both groups. 



