Topography of the Greenstone. 



417 



valley is a fine grained mixture of hornblende and feldspar. This is 

 sometimes columnar, as already described. Not unfrequently too, it is 

 amygdaloidal ; though the more perfect this structure, the less perfect 

 the columnar. Trap tuff is also much more frequent than I former- 

 ly supposed. Sometimes we find a mass of it overlaying a mass of col- 

 umns; and sometimes it forms an irregular layer between the ends of suc- 

 cessive stories of columns. A mile east of the village in Greenfield, a 

 variety of tuff constitutes a large part of a ledge of greenstone, which 

 in some places is more than a hundred feet high. I have observed 

 this rock on the west side of Mount Tom, also on Holyoke, and 

 in various other places. It must be carefully distinguished from the 

 trap conglomerate, that has been already described as a member of 

 the new red sandstone formation, lying upon the eastern side of 

 Mount Holyoke and Tom. The real trap tuff contains no fragments 

 of sandstone ; whereas in the trap conglomerate, the sandstone usual- 

 ly forms the cement. But the two rocks obviously pass into each 

 other. 



The boldness and wildness of the scenery in the Connecticut valley 

 result principally from the greenstone ridges that have been de- 

 scribed. The summits of these are often very irregular, owing to 

 the peculiarity of the mode in which they were produced, or to sub- 

 sequent abrasion by water. 



In those portions of the State that are composed of gneiss, particu- 

 larly in Worcester county, we frequently meet with bowlders of a 

 rock that appears when broken, exceedingly like the unstratified 

 greenstone that has been described. It is, however, more decidedly 

 crystalline in its structure ; yet I am inclined to believe that it ought 

 to be regarded as unstratified greenstone : certainly in Amherst, 

 where is a bed of considerable extent. The erratic blocks are often 

 two or even three feet in diameter ; and in the bed is no appearance 

 of stratification. In two instances, (in Rutland and near the mouth of 

 Miller's river, in Montague,) I have met with genuine veins of green- 

 stone in gneiss ; in neither case, however, of much width ; in the lat- 

 ter about two feet. 



Situation of our Greenstone in relation to other Rocks. 



This subject has been necessarily somewhat anticipated. But a 

 more particular statement of facts concerning it seems desirable. 

 It has already been stated that our unstratified rocks occur in three 



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