294 PENTTI ESKOLA 



malic minerals was found to show a certain regularity, the mica 

 always being richest and the clinopyroxene lowest in the iron 

 compound. The variation of the " Fe-quotient " is believed to offer 

 an important characteristic of crystalline rocks, though at present 

 little understood. 



When silica-bearing limestones are subjected to metamorphism 

 there occur reactions between the carbonates and silica, and 

 silicates of lime and magnesia are formed. The temperature of 

 reaction varies with pressure and is different for different minerals 

 formed, as pointed out by V. M. Goldschmidt. The writer's 

 earlier investigations have established that, among the common 

 accessory silicates in limestones, wollastonite requires the highest 

 temperatures to form, and diopside and tremolite, successively 

 lower. At still lower temperatures silica, in the form of quartz, 

 remains uncombined. Thus we may distinguish the following 

 types of metamorphic limestone: wollastonite limestone, diopside 

 limestone, tremolite limestone, and quartz limestone. These types 

 may be used, under certain conditions, as a geologic thermometer, 

 and it is hoped that the equilibrium curves of the different silicates 

 with the carbonates may soon be determined experimentally. 



The limestones of western Massachusetts were found to repre- 

 sent all the above-named types excepting the wollastonite limestone. 

 Their mode of occurrence harmonizes with the writer's earlier 

 experience, diopside limestone occurring at the immediate contacts 

 of the gneiss and tremolite limestone and quartz limestone 

 successively farther away. 



A review of the writer's experience from limestone-bearing 

 regions where intrusive granites occur seems to prove that such 

 phenomena of assimilation of limestone as those observed in 

 western Massachusetts are not at all of regular occurrence. Prefer- 

 ably they seem to occur in those regions where gneiss magmas 

 have been intruded in connection with mountain folding, thus 

 being dependent on the mechanical conditions in all probability. 

 It appears, also, that assimilation does not require very high 

 temperatures, being a very common phenomenon in granite 

 pegmatite cutting limestones. 



