GNEISS AND LIMESTONE CONTACT PHENOMENA 289 



under the conditions of stress and movements. The effective 

 stirring together of the earher solid materials with the newly- 

 intruded mass doubtless was one of the chief reasons why assimila- 

 tion of limestone on a rather large scale has occurred here. 



Very obviously some amount of assimilation of older silicate 

 rocks associated with the limestone has also taken place. Some 

 evidences to this effect have been presented by Emerson.^ In its 

 actual characters, however, the banded gneiss is altogether an 

 igneous rock and its band structure is not by any means a direct 

 result of injection combined with some kind of ultrametamorphism. 

 Its whole structure is markedly different from that of such injection 

 gneisses and migmatites known to me from many other regions 

 and, also, from the eastern boundary zone of the Becket gneiss 

 west of Chester. 



We may make some comparisons with similar phenomena in 

 other regions. Such clinopyroxene gneisses, or clinopyroxene 

 granites, occurring in connection with limestones and evidently 

 formed by assimilation of limestones surely do occur in many 

 tracts. That they are little noticed may in part be due to their 

 inconspicuous aspect and often imperceptible diff'erence in the 

 hand specimen from ordinary granites and gneisses. On the other 

 hand, it is quite certain that they are not by any means regular 

 associates of granites intruding limestones. I shall name some 

 examples from my own experience. 



My work in the Orijarvi region in southwestern Finland proved 

 the absence rather than the occurrence of such products of assimila- 

 tion of limestone, although it is an area where gneissic granites 

 are intrusive into limestone-bearing formations. Only in a few 

 places was the boundary type of the gneiss granite against limestone 

 found to be diopside-bearing,^ and only where it intersected lime- 

 stone as dikes. None of the granites designated as "microcline 

 granite," which are distributed all through southern Finland and 

 very often cut limestones or contain inclusions of them, was ever 

 found to contain clinopyroxene as a rock mineral, excepting in 

 pegmatite dikes (cf. below). Similar relations have also been 



' U.S. Geol. Surv., Bull. 597, p. 154. 



^ P. Eskola, Bull. Comm. geol. Finl., No. 40 (1914), p. 61. 



