THE DEERFIELD SHEET. 443 



the Deerfield River, but I have not met it on the north or in place. It is 

 a clear, light-gray rock, with roundish blotches of white, and it looks like 

 a weathered leucitophyre. Under the microscope the blotches are seen to 

 be made up of aggregated stout crystals of plagioclase, and the rest of tlie 

 mass between of rodlike plagioclase and magnetite, with almost no augite. 

 The rare amygdules in this rock are filled with a fine silky, radiated 

 mineral, apparently an altered prehnite resting upon diabantite, or more 

 rarely lined with glassy crystals of albite, with datolite, pyrite, or globules 

 of zincblende. 



RED DIOPSIDE-DIABASE, WITH SECONDARY ALBITE. 



Much of the basal part of the Deerfield bed just north of the Deei-field 

 River is a peculiar rock, remarkably different from the usual monotonous 

 ti'ap of the region. It has been radically metamorphosed by hot water 

 during its cooling. It is a fresh, fine-grained, brick-red rock, full of small 

 cavities and scattered larger ones, both lined or filled with exquisite albite 

 crystals large enough to be easily studied with a lens (fig. 24, C, p. 422). 

 The feldspars "of first consolidation" in the body of the trap, which are 

 near oligoclase, have been floated to their present place in delicate feathery 

 groups. They retain their sharp crystal outlines and trace of cleavage aud 

 multiple twinning on two bands, but have been changed to a sericitic mass 

 of subjiarallel scales and needles of two kinds, very minute needles polar- 

 izing in low colors COlS™"" long and 0.0003°"" wide, and brightly polarizing 

 scales 0.04°"" long. They seem to be kaolin and mica. 



The ordinary brown interstitial augite is wanting, but a few much 

 twinned idiomorphic diopside crystals occur. The above minerals are free 

 from the very abundant hematite which in grains and dendritic growths fill 

 the second generation of feldspars and make most of the slide opaque, and 

 which entirely replace the u.sual black ores and colored augites. The 

 second generation of feldspars is often in sheaves of parallel fibers, one or 

 more generally projecting far beyond the rest. They are heavily loaded with 

 the red rust, but often have clear borders or the rust is in a cross occupying 

 the diagonals to the square sections. 



Many of the cavities are filled with a fresh albite mosaic, and this at 

 times closely resembles the limpid feldspar mosaic of the amphil xylites, 

 being often without twinning and sliowing the same concentric polarizatioia. 

 This want of twinning is largely due to the development of the alljite in 



