Origin of Greenstone 



441 



tion generally. A similar case I have described on Mount Holyoke in 

 Belchertown. I have also given drawings of less extensive, though 

 not less decided cases of the agency of a mechanical force upon the 

 strata in juxtaposition with the greenstone, whereby a portion of 

 these strata is forced upwards, at Charlestown, Northampton, and 

 New Haven. All these facts, it seems to me, admit but one explana- 

 tion ; and irresistibly lead the mind to the conclusion, that a force 

 must have acted in the interior of the earth, urging the greenstone 

 through the superimposed rocks in a fluid or semi-fluid state. 



5. The Chemical effects of Greenstone upon the Stratified Rocks. 

 At Nahant we have seen that it has converted clay slate into flinty 

 slate : in Charlestown a similar change has taken place. In the 

 Connecticut valley, much of the sandstone in contact with the green- 

 stone has become vesicular and some of it is highly indurated ; and 

 in one case at least, somewhat columnar. Its red color too is often 

 destroyed, its texture rendered somewhat crystalline, and the mica- 

 ceous varieties partially changed to mica slate. In one case, also, 

 limestone is converted into tripoli : that is, its carbonic acid is ex- 

 pelled. 



Now it requires no labored argument to show that such effects as 

 these could have resulted only from the intrusion among the strata of 

 rocks in a state of fusion, or intensely heated. It does not require 

 even a practised geologist to draw this conclusion: for the facts, 

 wherever they exist, impress every man who observes them with this 

 belief. 



Upon the whole I cannot see that any thing is wanting to prove 

 the igneous origin of our greenstone. It may be asked, indeed, how 

 it happens, that while existing volcanoes throw up their matter in a 

 conical shape, greenstone forms a continuous ridge, 70 or 80 miles 

 long, with no appearance of a crater or craters. There is reason to be- 

 lie\e, indeed, that the mode in which greenstone was erupted, was con- 

 siderably different from the operation of existing volcanoes ; and that 

 probably the protrusion took place under an immense weight of water : 

 nor can it be imagined how a common volcanic force, which acts in the 

 direction of the radii of a circle, should thus operate lineally. But it 

 is easy to conceive how the shrinking of the interior part of the earth 

 by refrigeration, faster than the exterior, would produce such linear 

 openings, into which the melted matter from beneath would readily 

 force its way. But more of this ingenious hypothesis in the sequel. 

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