264 



Scientific Geology. 



struction of Fort Adams, we see frequent examples of a brecciated 

 or conglomerated structure. It is also traversed by numerous small 

 veins of white quartz, sometimes combined with flesh colored feldspar. 



The serpentine is separated from the granite by a strip of flinty 

 slate. At its eastern extremity it seems to lie between the flinty slate 

 and the graywacke slate, and to have a stratified structure. But it 

 probably extends to the southwest, (as shown on the sketch by the 

 crosses,) so as to cut across the northwestern point of the siliceous 

 slate. A valley passes through the flinty slate in the direction in 

 which the serpentine runs, and at its extremity, serpentine appears in 

 small masses attached to the flinty slate. It probably forms a sort 

 of vein in the slate, though hid by the loose soil, and at its north- 

 eastern extremity the graywacke slate lies immediately north of it, 

 as shown on the preceding sketch. I am inclined to believe, how- 

 ever, that the serpentine was originally interstratified with, or formed 

 a bed (if that term conveys any definite meaning,) in grayAvacke 

 slate : though the extreme degree of contortion in the slate, where 

 the two rocks join, renders it not easy to decide that point. The 

 serpentine is compact, very hard, and of a very dark color. It might 

 easily be mistaken for greenstone. 



The limestone forms a small island, a little distance north of the 

 serpentine ; also a small point projecting into the harbor, near Fort 

 Adams. It is entirely destitute of stratification, is perfectly compact, 

 and nearly as hard as quartz. Its general color is a grayish white ; 

 but it abounds in gray spots, which resemble chert. (No. 495.) In- 

 deed, the whole mass seems to be well advanced in the process of con- 

 version into that substance. This seems to be the case referred to by 

 Dr. Macculloch, when he says : " an attempt to the production of this 

 rock (chert) is often observed where the process is still incomplete : 

 and it is evinced by the extreme hardness which such limestones ex- 

 hibit in the vicinity of granite."* He refers here to the primary 

 limestones, which are intermingled with siliceous and argillaceous 

 matter : and that the limestone at Newport is primary, in the sense 

 in which Dr. Macculloch uses the term, is evident from the fact, that 

 near the serpentine, a portion of it is seen in the form of a bed, be- 

 tween the layers of graywacke slate : and the limestone associated 

 with this rock, is precisely the kind that is apt to contain a consider- 

 able proportion of siliceous and argillaceous matter. In ordinary 

 cases, limestone is rendered more crystalline by the proximity of 



* System of Geology, Vol. 2. p. 285. 



