GENESIS OF THE LITCHFIELD ROCK. 239 



The powder that fell at 2.56 contains some untwinned grains and many 

 with the twinning striations of microcline. Its composition is given under 

 column II. As will be seen by comparison with the figures for ortho- 

 elase, this mineral also is very pure. There can be no doubt that it is a 

 potassium feldspar, and it is probable that it crystallizes in both mono- 

 clinic and triclinic forms. 



In view of the fact that eleolite-syenite is defined as a rock consisting 

 essentially of orthoclase and eleolite, it becomes of importance to determine 

 whether the potash feldspars in the Maine rock arc primary or secondary.. 

 It is very evident that they are younger than the eleolite and the large 

 crystals of albite, and are of the same age as the albite grains in the 

 mosaic. Their small grain, perfect transparency, lack of cleavage, and 

 the method of their occurrence in narrow stringers and small areas be- 

 tween the undoubted primary constituents point to a secondary origin 

 for all the minerals in the mosaic. The arrangement of these is, however, 

 somewhat peculiar, in that in nearly every ease they are more or less 

 lenticular and their long axes are rudely parallel to the long directions 

 of the areas which they form. This would indicate that the pressure by 

 which the rock was made schistose acted after the feldspar grains of the 

 mosaic were formed. The explanation of the phenomenon seems to be 

 that the rock which originally consisted of eleolite, albite, lepidomelane, 

 and perhaps some orthoclase or other feldspar, was subjected to great 

 pressure intended by motion, that it was broken and shattered, and 

 that the fragments were rolled upon one another, and at the same time 

 albite and orthoclase were deposited in all the crevices as they were 

 formed. The pressure and motion continued until all the newly formed 

 grains became oriented, and some had developed in them twinning 

 lamella'. From all the evidence at hand it would appear that the micro- 

 cline in the Litchfield rock is merely an orthoclase with secondary cross- 

 twinning. 



An indication of the correctness of this view is the fact that where the 

 feldspathic mosaic is absent the rock is massive and not schistose — i. < .. 

 where pressure has not produced foliation there is an absence of the small 

 grains of feldspar composing the mosaic. 



The only two constituents remaining to lie described are cancrinitc 

 and sodalite. The latter may usually he recognized by its lighl blue 

 color in natural light, though at times its tint is so pale that it can he 

 detected only by the contrast afforded by the colorless minerals associ- 

 ated with it, which appear to be slightly tinged with yellow, Under 

 crossed nicols it is perfectly isotropic. No idiomorphie forms occur, bul 

 the substance extends irregularly around the other components includ 

 iic: them, as augite does the feldspar in many diabases. The most abun- 



\ \ X 1 1 I'.i i i 1. 1. ii Sin \ >i \ "i . ::. 1891, 



