PETROGRAPHY OF THE GREENFIELD BED. 71 



The absorption in this mineral is very strong : a = blue, fc = yellowish 

 green, c = brownish yellow. A single twin with an extinction of 38 de- 

 grees on either side of the suture was found, and the maximum of the 

 blue absorption was also at 38 degrees on either side of the suture, and 

 this blue absorption represented the greatest elasticity. The mineral 

 thus has the positive sign and the strong absorption of segyrite and the 

 optical figure in the position of augite. The mineral is thus nearer 

 regyrite than the segerine-augite of Rosenbusch, in that the absorption 

 parallel to a is clear blue and not grass green. Large patches of the 

 mineral are changed to a yellow green serpentinous mineral, which under 

 crossed nicols is almost black, but with scattered points of light. 



The diopside is in stout small crystals or in long stout prisms, some- 

 times broken. They are enveloped by the segerine-augite without com- 

 mon orientation. 



Amygdaloidal sandstone. — One of the columns of sand rising from the 

 sandstone and penetrating the basal bed at the Greenfield quarry expands 

 nine feet from the base where it passes above the basal bed into the glass 

 breccia, and its central portion presents a scoriaceous appearance. It is 

 a red sandstone filled with more or less rounded spots of a white silicate 

 which I have no doubt, from my examination of other similar cases, is 

 mainly a granular plagioclase. The same thing is developed much more 

 extensively on the path going up over the cliff north of the quarry. 

 Here for several feet in thickness the rock is a red sandstone closely filled 

 with small cavities. The whole makes the impression of a rather coarse 

 red amygdaloid with white amygdules. 



A still more attractive form of the same rock is found in the cut of the 

 electric road at the Deerfield river, a mile south of Cheapside (see plate 7, 

 figure 3). Here a light red sand rock is filled with the fresh white 

 amygdules, producing a very attractive rock. Under the microscope the 

 sandstone between the white fillings has a beautiful fluidal structure, thus 

 heightening the resemblance to an amygdaloid. The cavities are super- 

 ficially blackened by the recrystallization of the iron oxide. The white 

 filling is mainly a fresh matted network of plagioclase blades, which 

 shows distinct triclinic striation, rather more frequently than is usual in 

 this water-deposited feldspar. They are ragged edged from interference 

 due to rapid crystallization. In the center of the cavities is another min* 

 eral into which the feldspars penetrate with a micropegmatitic structure 

 or which runs out among them. It polarizes with bright yellows, and I 

 suspect it to be datolite, as a mineral with the high glassy luster of dato- 

 lite can be seen with the lens in the centers of some cavities. It may be 

 diopside, but shows no cleavage, and it has a rough surface like olivine, 

 which agrees with the high refractive index of datolite. Another pecu- 



