292 PENTTI ESKOLA 



The pegmatites, like other igneous rocks, show a varying 

 behavior toward the limestones: The writer has seen, in several 

 limestone areas, swarms of pegmatite dikes some of which carry 

 clinopyroxene and titanite whereas others do not and could find 

 no differences to which this might be ascribed. 



The common occurrence of phenomena of assimilation in 

 pegmatites interests us because it proves decisively that this 

 process does not require any high temperatures. The pegmatites 

 crystallize out from residual solutions which still exist in liquid 

 form after the main igneous bodies have become solid. The 

 common occurrence of j8-quartz as well as a-quartz proves that 

 the temperature has frequently been below their transformation 

 point at 575° during the crystallization of pegmatites. 



Now, what has been the temperature of consolidation of the 

 diopside-bearing varieties of the Becket gneiss? If we could 

 estimate this, we would know that the assimilation took place at 

 slightly higher temperatures than that of the beginning of crystalli- 

 zation. This must have happened between the three-phase points 

 quartz-calcite-wollastonite and quartz-dolomite-diopside invariant 

 under the existing pressure. The higher of these points apparently 

 lies below the inversion-point a-quartz-tridymite, but at present 

 we cannot state it more closely. 



The other minerals of the gneiss indicate that the temperature 

 had decreased very much before crystallization was complete. 

 The rock contains epidote as individual large grains associated 

 with albite, the epidote probably not being entirely of secondary 

 origin. The potash feldspar does not contain any threads of 

 albite (perthite), a fact that might indicate that the temperature 

 was so low when the microcline crystallized that no considerable 

 amount of albite could be taken into solid solution to separate later 

 and form perthite. Only the low est- temperature pegmatites 

 contain such homogeneous potash feldspar. 



Turning finally to the question why limestone is assimilated 

 in some cases and in others not at all, it seems that this may be 

 largely dependent upon mechanical conditions. Assimilation is 

 promoted, if the intrusion is connected with folding and the intrusive 

 magmas are agitated and mixed with the crushed materials from 



