Chemical Effects of Greentsorie. 



429 



to 90°. Must we not impute this flexure to the protrusive force of the 

 greenstone, when first it was elevated to the day light? 



Junction of Greenstone and Sandstone : Mount Tom. 



Chemical Effects of Greenstone upon other Rocks. 



In other parts of the world, it is a common case to find the rocks 

 lying in contact with greenstone, essentially changed in their charac- 

 ters, for a greater or less distance from the place of junction. This 

 is most striking where limestone is the rock invaded by the trap. 

 Similar effects are not wanting in the rocks of Massachusetts, that 

 are traversed by greenstone. Yet it appears to me that they are 

 hardly as common or striking as in some countries ; judging from 

 the descriptions of geologists. One reason may be that greenstone 

 here rarely comes in contact with limestone. The following are the 

 principal examples of this phenomenon which I have met with. 



The influence of greenstone veins at Nahant, in converting argil- 

 laceous slate into flinty slate, and where carbonate of lime was pres- 

 ent, into chert, has been fully described under gray wacke. 



Professor Webster describes a mass of trap, in Charlestown, as 

 superincumbent upon a rock which he calls compact feldspar, "which 

 has many of the characters of clay slate, and in the immediate vicin- 

 ity of the trap rock has a degree of hardness, a compact structure, 

 and fracture almost like that of hornstone, — the slate seems to have 

 undergone a great and remarkable change."* 



In the Connecticut valley the most striking chemical effects pro- 

 duced upon the sandstone by the greenstone, are induration, a vesic- 

 ular structure, and change of color. In the 17th volume of the Am. 

 Journal of Science, Professor Silliman has described a most interest- 

 ing example of all these effects, as they appear in a quarry, nearly a 



* Boston Journal of Philosophy, Vol. i. p. 282. 



