KNOWLEDGE OF EARTH STRUCTURE 161 



this question. To President Cooper especially, who regards the 

 greenstone of the Connecticut as volcanic, I feel much indebted 

 for the great mass of facts he has collected on the subject. And 

 were I to adopt any hypothesis in regard to the origin of our 

 greenstone, it would be one not much different from his" (p. 60). 



By 1833 and more clearly in 1841 Hitchcock had come 

 to recognize the distinction between intrusive and extru- 

 sive basaltic sheets in the Connecticut valley. Dawson 

 also came to regard the Acadian sheets as extrusive, and 

 Emerson in 1882 recalled again the evidence for Massa- 

 chusetts (24, 195, 1882). Davis, however, went a step 

 further and by applying distinctive criteria not only sep- 

 arated intrusive and extrusive sheets throughout the 

 whole Triassic area, but by using basalt flows as strati- 

 graphic horizons unraveled for the first time the system 

 of faults which cut the Triassic system. His preliminary 

 paper (24, 345, 1882) was followed by many others. 



From 1880 onward begins the period of precise struc- 

 tural field work. The older geologists mostly conceived 

 their work after reconnaissance methods. From 1870 to 

 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. 



Views 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 



