Relative Situation of Greenstone. 



419 



stone results from the same source as hornblende slate, it may be diffi- 

 cult in some cases to distinguish between them. 



Professor Webster, in his excellent account of the geology of the 

 region around Boston, states that the veins of greenstone in the gray- 

 wacke conglomerate of that vicinity, run about 10° W. of South, and 

 10° E of North. All such veins are probably of nearly cotemporary 

 origin: their parallelism being explicable only on the supposition of 

 their having been produced by the same cause. 



The promontory of Nahant presents an interesting exhibition of 

 greenstone veins, both in the argillaceous slate and the sienite. I 

 have, however, described these veins so fully under graywacke, that 

 a few more facts only need be added. Only a small remnant of the 

 slate remains upon this promontory : and this is intersected by so ma- 

 ny and so large veins, that nearly one half of the surface is green- 

 stone. And yet the layers of the slate appear to have been but little 

 thrown out of their original position: for their dip and direction cor- 

 respond essentially with those of the same rock in other places. In 

 such cases it seems to me impossible that the slate should have been 

 solid at the time the greenstone was intruded among it, unless we 

 suppose it to have been cut through wirh numerous fissures : an oc- 

 currence which in the present case is hardly probable ; since some of 

 the veins are ten feet thick, and quite numerous ; and I cannot con- 

 ceive how mere dessication should have produced such fissures. But 

 I can conceive how melted matter may have been forced through un- 

 consolidated clay, without disturbing it laterally but a short distance : 

 and perhaps this was the mode in which the veins at Nahant were 

 introduced. If so, it is probable that the consolidation of the slate, and 

 even its conversion into flinty slate, might have resulted from this in- 

 trusion. 



There is one fact, however, that rather militates against such a sup- 

 position. We find there two sets of veins; one of which intersects 

 the other.; and penetrates the adjoining slate. We here trace distinct- 

 ly three epochs of formation for the slate and greenstone. First the 

 slate, secondly the veins that intersect it, and are themselves intersect- 

 ified by other veins: thirdly, those veins that cross both the first nam- 

 ed veins and the slate. As to the intervals between the production of 

 these three varieties of rock, we can scarcely form a conjecture. The 

 slate having been deposited originally from water, must have requir- 

 ed a period of considerable length, previous to its consolidation : But 

 "he two sets of veins might have been introduced almost simultaneous- 



