56 EOCKS FOEMED THEOUGH IGNEOUS AGENCIES 



acted on any one rock mass as to expose superficial and deep- 

 seated portions alike. In those regions of greatest geological 

 antiquity, erosion has removed more or less completely the 

 superficial parts and left for our inspection those portions of 

 a magma that at the time of eruption never reached the sur- 

 face, but cooled, it may be, under thousands of feet of super- 

 incumbent matter. Such rocks are as a rule more highly crys- 

 talline than those which flowed out upon the surface like the 

 modern lavas. Hence it is that from a very early period it 

 has been found convenient to divide the eruptive rocks into two 

 general groups: first, the intrusive or plutomc rocks; and sec- 

 ond, the effusive or volcanic rocks. 



Although this classification has not been strictly adhered to 

 in the present work, a few words descriptive of the essential 

 distinctions between plutonic and effusive rocks will not be out 

 of place, since such distinctions, particularly in eroded regions, 

 afford the only criteria for discrimination as to the original 

 conditions under which a rock mass has been formed, and hence 

 are of value in the field. 



As a general rule, it may be said that the structural features 

 of an eruptive rock depend upon the conditions under which 

 a magma has cooled, although undoubtedly the amount of 

 included vapor of water may exert a powerful influence. As 

 Professor J. P. Iddings has well expressed it, ^'the chemical 

 differences of igneous rocks are the result of a chemical differ- 

 entiation of a general magma, and the structure of a rock is 

 dependent upon the physical conditions attending its eruption 

 and solidification." Now it is at once apparent that the greater 

 the depth below the surface at which a magma undergoes 

 solidification, or the greater its mass, the slower, more gradual, 

 will be that solidification, and hence the more complete and 

 coarser will be the crystallization. Hence the strictly plutonic 

 rocks are always holoerystalline. And, inasmuch as the weight 

 of the superincumbent matter has been such as to prevent the 

 expansion of included vapors to form steam cavities, so these 

 rocks are never vesicular or pumiceous, but compact and gran- 

 ular throughout. In cases where a plutonic rock has been 

 voided upward to fill a pre-existing rift in the form of a dike, 

 those portions of the magma coming in contact with the cold 

 walls on either hand will cool most quickly. Hence a dike is 

 most coarsely crystalline near the centre, and finer grained, per- 



