62 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



be required to remove the original surface phase. The geologist 

 studying the erosion surface might have no inkling that the " batholith " 

 had not been completely covered by a roof of country-rock. The 

 former existence of a roof cannot be assumed simply because a " batho- 

 lith " has a holocrystalline structure. 



^ooc/gc/ /4/-)9c 



/^,-ff£f (y/ Hoof rocM W/f/r o^normo/ 



. . , / A /her/no/ Orac/te/?/ 



PHA3^ Of 



Figure 2. Ideal section of a district of batholithic foundering. Length 

 of section assumed, for example, as 60 kilometers. 



Blue Hills, Massachisetts. 



The relations of the granite, quartz porphyry, and aporhyolite of the 

 Blue Hills complex, near Boston, Massachusetts, to each other and to 

 the Cambrian slate which forms the country-rock of the intrusive, 

 suggest at least that the batholithic roof was very thin. Crosby de- 

 scribes two large areas of the aporhyolite (felsite) with the characters 

 of both intrusive and effusive magma. He favors the view that these 

 bodies are very thick surface flows of the ordinary tj'pe, though he 

 states the alternative view that the aporhyolite represents the intru- 

 sive, latest phase of the batholithic magma. ^^ The present writer 

 believes that, in this case, the hypothesis of partial roof-foundering is 

 worthy of a competitive place in a full discussion. The belief is founded 

 on actual acquaintance with only a limited part of the observable field 

 relations, and it is offered merely as a preliminary suggestion. 



Glen Coe, Scotland. 



The noteworthy paper of Clough, ]\Iaufe, and Bailey on " The 

 Cauldron-subsidence of Glen Coe," illustrates a case where part of a 



^* W. O. Crosby, The Blue Hills Complex. Occasional Papers of the Bos- 

 ton Society of Natural History, 4, 385 (1900). 



