I. EOCKS FORMED THROUGH IGNEOUS AGENCIES. 



ERUPTIVE 



This group includes all those rocks which having been once 

 in a state of igneous fusion have been forced upward and in- 

 truded into the overlying rocks in the form of bosses, laccoliths, 

 dikes, and sheets, or poured out upon the surface as lavas. 



Concerning the source of eruptive rocks we are yet in igno- 

 rance. In times past they have been supposed by many to repre- 

 sent portions of the still unconsolidated interior of the earth. 

 The great variety of igneous rocks, the wide variation in chemical 

 composition as well as the apparent independence of closely 

 adjacent volcanoes, both in the matters of time of eruption and 

 character of erupted material, seem, however, to show that they 

 come not from a common reservoir, but from isolated and com- 

 paratively small areas where, for reasons not now well under- 

 stood, previously solidified rock masses have been so highly heated 

 as to become pasty or liquid; and then, through their own ex- 

 pansion, or that of included vapors, or by compressive forces 

 generated in the earth's crust, forced upward into the positions 

 they now occupy. The origin of igneous rocks belongs as yet 

 to the realm of speculation. We must here confine ourselves 

 more to their mineral and chemical nature, general physical 

 properties, and the conditions under which they occur. 



Consider, then, a mass of molten rock material, — to which 

 the term magma may be conveniently applied, — and which by 

 the processes of eruption is forced upward toward the surface, 

 and let us dwell briefly upon the forms assumed by this magma 

 on cooling under the various conditions in which it finds itself. 

 It is obvious at the start that we can have actually to do with 

 but a comparatively limited portion of the products of any erup- 

 tion. If the molten material is poured out upon the surface and 

 there remains for inspection to-day, it is a necessary consequence 

 that the deeper-lying portions are obscured. If, on the other 

 hand, the superficial portions have been removed by erosion so 

 as to expose the deeply lying parts, we have only the latter for 

 study and observation. It is rare indeed that erosion has so 



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