Topography of the Greenstone. 



413 



the gray wacke and clay slate, we usually meet with narrow masses 

 of greenstone, probably in some cases interposed among the layers 

 of slate. On the north of Boston, in the slate of Charlestown partic- 

 ularly, such masses or beds of greenstone are common ; and some of 

 them so large that I have noted two of them on the map. In this 

 slate also, as well as in the gray wacke in other places, (as at Roxbu- 

 ry,) the greenstone is found in veins. At Nahant they are sometimes 

 forty feet thick, in argillaceous slate and sienite. 



If we proceed from Boston, after striking the deposit of greenstone 

 above named, we shall soon find that it is passing into sienite, and 

 mixed with sienite in almost every conceivable mode. In one place 

 the greenstone seems to form a distinct vein in the sienite, the two 

 rocks being well defined at their line of junction. In another place, 

 the sienite seems to form veins in the greenstone ; although in such 

 cases it is no easy matter to determine which rock should have the 

 posteriority: But from the general fact, which I think obvious in this 

 region, that the greenstone has been produced subsequently to the 

 sienite, I think we should be cautious in reversing this order without 

 good evidence. In some cases, however, we meet with a reddish 

 sienite containing numerous and sometimes large angular and round- 

 ed fragments of greenstone. I. give a rough sketch below, of one of 

 these cases, which I observed in Marblehead, a little west of the town. 

 In this case the base of the rock is rather a red granite than sienite, 

 being entirely destitute of hornblende. 



Granite and Greenstone : Marblehead. 



Instances similar to this are to be seen every where in the region 

 under consideration. And they certainly appear as if the greenstone 

 had been partially melted down in the granite ; though the heat was 



