GNEISS AND LIMESTONE CONTACT PHENOMENA 285 



Tremolite limestones correspond with transitional forms 

 between the greenschist facies and another facies called the amphib- 

 olite facies J because a rock of gabbroid composition whose minerals 

 have formed under the conditions of this facies, appears as a 

 plagioclase-hornblende rock, an amphibolite. 



The diopside limestones, so far as known at present, also 

 belong to the amphibolite facies, and even some of the wollastonite 

 limestones belong here, but the last named cover a wide range of 

 temperature and pressure conditions corresponding to several kinds 

 of mineral development in the silicate rocks, and a gabbroid 

 material may give any one of the mineral combinations : plagioclase 

 and clinoenstatite-diopside solid solutions (provisionally referred to 

 as the sanidinite facies) or plagioclase, diopside, and hypersthene 

 {hornfels facies) , or diopside-jadeite solid solutions and pyrope- 

 almandite solid solutions {eclogite facies) . 



Metamorphic limestone as a geologic thermometer can be 

 calibrated against the transformation points of the silica minerals. 

 The fact that quartz, and not tridymite, occurs even with wollasto- 

 nite, would seem to indicate that the wollastonite-curve passes 

 below the transformation point quartz-tridymite. This is, accord- 

 ing to Fenner,^ 870°, but it must rise somewhat with pressure. 

 As to the transformation point Q:-/3-quartz at 575°, it has been 

 established that crystals of quartz in a diopside limestone from 

 Parainen in southern Finland, have been originally o;-quartz,^ 

 while the quartz-dolomite rocks of eastern Finland contain clear 

 crystals of quartz which is apparently primary /S-quartz. More 

 observations on this subject are desirable. 



It is to be hoped that the temperature-pressure curves for the 

 different siHcate-carbonate equilibria may be determined exactly by 

 experiment, in the near future. 



In the contact zones of the Becket gneiss in western Mas- 

 sachusetts three of these paragenetic types of limestone occur, 

 namely: quartz limestone, tremolite limestone, and diopside lime- 

 stone. The temperature was probably nowhere high enough to 



^ C. N. Fenner, Amer. Jour. Sci., Vol. XXXVI (1913), p. 331. 

 ^ A. Laitakari, Bull. Com. geol. Finl., 54 (1920), p. 40. 



