Ban-ell — Growth of Knowledge of Earth Structure. Ill 



1880 a group of younger men entered geology who paid 

 close attention to. the solid geometry and mechanics of 

 earth structures. In their hands physical and dynamical 

 geology began to assume the standing of a precise and 

 quantitative science. In the field of intrusive rocks the 

 opening classic was by Gilbert, who in his volume on the 

 geology of the Henry Mountains, published in 1880, made 

 laccoliths known to the world. With the beginning of 

 this new period we may well leave the subject of intru- 

 sive rocks and turn to the progress of knowledge in 

 regard to those deeper and vaster bodies now known as 

 batholiths. These, since erosion does not expose their 

 bottoms, Daly separates from intrusives and classifies as 

 subjacent, The batholiths consist typically of granite 

 and granodiorite, and introduce us to the problem of 

 granite. 



Vieius on the Structural Relations of Granite. 



Conscientious field observations were sufficient to 

 establish the true nature of the intrusive and extrusive 

 rocks. The case was very different, however, with the 

 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. 



