GNEISS AND LIMESTONE CONTACT PHENOMENA 267 



Numerous questions as to why and how the contact rocks have 

 obtained their present characters arise from such studies, but to 

 many of them no definite answers are yet possible. It often seems 

 wiser to direct our efforts toward finding a sound form of the 

 question, according to Bacon's advice: "Bene interrogatum quasi 

 dimidium responsi." These questions, one may hope, will be 

 cleared up either by future experimental investigation or by 

 accumulating more evidence from natural rocks. 



The questions to be discussed in the present paper are: Why 

 is limestone sometimes assimilated by magmas, and sometimes 

 not? What are the factors controlling the formation of silicate 

 minerals in limestone during metamorphism ? What laws control 

 the distribution of elements, such as Mg and Fe, which replace each 

 other in solid solutions, among different mineral groups, pyroxenes, 

 amphiboles, and micas, in silicate rocks ? 



I wish to express my best thanks to Dr. N. L. Bowen for pleasant 

 companionship during the excursions and for many discussions, 

 and especially for his trouble in making the necessary grammatical 

 corrections in my writing. 



This work has been carried out with pecuniary assistance from 

 the Geophysical Laboratory of the Carnegie Institution of Wash- 

 ington and from two funds for the advancement of scientific 

 research in my native country, Finland, namely, Alfred Kordelin's 

 General Trust for the Advancement of Progress and Knowledge, 

 and Herman Rosenberg's Travelling Bursaries Trust of the Uni- 

 versity of Helsingfors. 



BECKET GRANITE GNEISS 



Emerson^ gives the following description of the Becket granite 

 gneiss : 



It is a medium to fine grained light-colored biotite (or biotite-muscovite) 

 microcline-oligoclase gneiss, with microscopic epidote uniformly blended with 

 the scanty biotite, and the microcline grains commonly grouped as if made of 

 the crushed fragments of larger porphyritic crystals. There are smaU areas 

 of light-colored porphyritic granite from which the prevalent rock could have 

 been produced by crushing. A micrographic texture is common. Over large 



' U.S. Geol. Surv., Bull, ^gy, p. 154. 



