162 A CENTURY OF SCIENCE 



nature and relations of the great bodies of granite, 

 which may be taken in the structural sense as including 

 all the visibly crystalline acidic and intermediate rocks, 

 known more specifically as granite, syenite, and diorite. 



The large bodies of granite, structurally classified as 

 stocks, or batholiths, commonly show wedges, tongues, or 

 dike networks cutting into the surrounding rocks. The 

 relations, however, are not all so simple as this. Gran- 

 ites may cover vast areas, they are usually the older 

 rocks, they are generally associated with regional 

 metamorphism of the intruded formations, which meta- 

 morphism is now understood to be due chiefly to the heat 

 and mineralizers given off from the granite magma, asso- 

 ciated with mashing and shearing of the surrounding 

 rocks. The granite was often injected in successive 

 stages which alternated with the stages of regional mash- 

 ing. A parallel or gneissic structure is thus developed 

 which is in part due to mashing, in part to igneous injec- 

 tion. Where the ascent of heat into the cover is exces- 

 sive, or where blocks are detached and involved in the 

 magma, the latter may dissolve some of the older cover 

 rocks, even where these were of sedimentary origin. 



Thus between mashing, injection, and assimilation the 

 genetic relationships of a batholith to its surroundings 

 are in many instances obscure. Nevertheless, attention 

 to the larger relations shows that the molten magma orig- 

 inated at great depths in the earth's crust, far below the 

 bottoms of geosynclines, and consists of primary igneous 

 material, not of fused sediments. From those depths it 

 has ascended by various processes into the outer crust, 

 where it crystallized into granite masses, to be later 

 exposed by erosion. The amount of material which can 

 be dissolved and assimilated must be small in compari- 

 son with the whole body of the magma. The original 

 composition of the magma was probably basic, nearer 

 that of a basalt than that of a granite. Differentiation 

 of the molten mass is thought to cause the upper and 

 lower parts of the chamber to become unlike, the lighter 

 and more acidic portion giving rise to the great bodies of 

 granite. With the exception of certain border zones the 

 whole, however, is regarded as igneous rock risen from 

 the depths. 



