Protrusion of Greenstone. 



421 



usually un stratified, should be regarded only as examples of a con- 

 extionary structure. 



The geologist who may have occasion to spend several days at 

 Nahant, will do well to give the spot a very thorough examination. 

 I do not flatter myself that 1 have brought to light all the interesting 

 facts which may be there developed ; although I have exhibited 

 enough to show it to be an interesting spot. 



The protrusion of the unstratified rocks through the stratified ones 

 by internal igneous agency, now admitted by most geologists, has led 

 observers to examine carefully for evidences of mechanical disturb- 

 ance near the line of contact. They have, I believe, found less proof 

 of such disturbance by the intrusion of greenstone, than in the case 

 of the older rocks, as sienite and granite. Every such case, there- 

 fore, deserves to be noticed. If I mistake not, the following sketch 

 of a vein of greenstone in argillaceous, slate is an example of this 

 sort. The dyke is about 10 feet thick, and the general dip of the 

 layers'of slate in the quarry, is about 30° southeast. But as shown in 

 the figure, near the greenstone it is considerably curved upwards in 

 the contrary direction. The quarry, where this example occurs, is 

 about half a mile north of the Powder House in Charlestown. 



Greenstone Dyke in Clay Slate : Charlestown. 



For the most part, the greenstone in the valley of the Connecticut, 

 is interposed in thick masses or beds, between the strata of sandstone. 

 In Massachusetts I have never met with a mass of this rock which I 

 have regarded, strictly speaking, as superincumbent : though in Con- 

 necticut, such examples are said frequently to occur. And I am ap- 

 prehensive that not a few geologists would describe Holyoke and 

 Tom as instances of overlying rocks. For on examining their west- 

 ern precipitous faces, we frequently find the sandstone cropping out 

 beneath the greenstone : and if we go to the other side of these 

 ridges, it is rare that we find the sandstone lying upon the greenstone. 

 Yet this is sometimes the case ; and the sandstone always appears 



