LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 85 



especially true of batholiths which crystallized at no great depth 

 from the surface and therefore against cold rocks. 



At greater depth, however, the magma is likely to have greater 

 effect on its country rock. The surrounding rock may become 

 thoroughly impregnated with magmatic material.^ Blocks of the 

 country rocks which have become broken off may become similarly 

 impregnated and a gradual mixing of the materials at their borders 

 with the magma may bring about finally a homogeneous mixture of 

 the country rock and magma. If this crystallizes as such, it gives 

 a hybrid rock. When the rock invaded is an igneous rock the 

 hybrid may be identical with a normal igneous rock intermediate 

 in composition between the two. If the two rocks involved lie 

 far apart in the igneous series, say granitic magma invades peri- 

 dotite, a hybrid of composition not represented among normal 

 igneous rocks may be formed. When the rock invaded is a sedi- 

 mentary rock certain minerals normally belonging to the meta- 

 morphosed sediment, garnet for example, may appear also in the 

 hybrid.^ 



However, the formation of an obviously hybrid rock should, 

 apparently, be the normal result of assimilation. Some petrologists 

 have assumed that as a result of assimilation the tendency toward 

 differentiation is so increased that the free differentiation which 

 follows destroys the simple hybrid relation. This opinion is 

 based in part on the assumption that rocks mutually lower each 

 others' crystallization temperatures. As a matter of fact, if two 

 rocks were mixed the temperature range of solidification of the 

 mixture would in general be intermediate between the corresponding 

 range of the individual rocks. This refers of course to the tempera- 

 ture range of solidification if no crystals are removed. If crystals 

 are removed the temperature range of crystalhzation of the syn- 

 tectic magma will be continually extended precisely as the tempera- 

 ture range of the original magma would have been extended by the 

 same process. The question of the effect of assimilation on differ- 

 entiation really reduces itself, then, to the question of its effect 



^ C. N. Fenner, "The Mode of Formation of Certain Gneisses in the Highlands of 

 New Jersey," Jour. GeoL, XXII (1914), 602. 

 ^ Cf. Harker, op. ciL, p. 338. 



