366 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



sandstone, with many quartz, musco\'ite, and grajjliite grains and scales 

 visible to the eye. It is tliick-bedded or massive, changing suddenly from 

 the thin-bedded sandstone. Its full thickness is not exposed. It is very 

 calcareous, and I think it possible that it may be the bed of limestone marked 

 near here by President Hitchcock on his maps, which I have not before 

 found so far north. 



In this calcareous sandstone are small rootlike concretionary Ijodies, 

 which appear in the rusted rock as minute tubes rarely branching. They 

 reach one-half inch in length, and at most one-eighth inch in diameter. In 

 the fresh rock they appear as white calcareous bodies, with a trace of longi- 

 tudinal fibrous structure, remotely suggesting a minute branching chaetetes. 



In this sandstone are many wholly angular fragments, from 1 to 4 

 inches long, of a volcanic rock, which may often make up a quarter of the 

 mass of the Avhole bed. In fi-esh fracture it is a white or light-gray, fine- 

 grained rock, with exactly the look of a somewhat siliceous linaestone 

 spotted with small grains of pyrite. Weathering or careful study with the 

 lens lirings out the fact that the rock is amygdaloidal, with small cavities, 

 mostly spherical, which are filled with calcite or pyrite, or both, and rarely 

 the reflection of a minute twinned plagioclase lath can be seen in the solid 

 rock. In a thin slide it is found to be a diabase considerably altered, but 

 preserving a close resemblance in man^s' particulars to the Mount Holyoke 

 trfip, but more to the abnormal red trap from Cheapside. (See p. 431.) It 

 has the same distant feathei-y groups of larger plagioclase of first consolida- 

 tion (0.8"'" long), just A'isible to the eye and containing rounded inclu.sions 

 of glassy magma, and these lie in an ophitic network of plagioclase laths 

 of two sizes, the one in quite stout rods, 0.4""" long, which are scattered 

 abundantly in a reticulate or stellate ground consisting of very fine needles 

 of plagioclase, 0.03-0.04°"" long. Both the finer feldspars are distinctly 

 fibrous, a structure Avhich is caused by lines and rows of minute grains of 

 a dark ore, which is doul^tless limonite, and was originally hematite, as in 

 the Cheapside trap. This makes up almost the whole content of iron in the 

 rock, as only one uncertain augite grain could be detected. There is no 

 magnetite or chloritic decomposition product except a trace of an amorphous 

 green constituent in the amygdules. 



To cmnplete the i-esemblance to the Cheapside rock, the small round 

 cavities are lined by a secondary growth of fresh albite in well-shaped 



