Graywacke Formation. 



257 



one of the oldest of the gray wacke formation. Its position in respect 

 to the granite is proof of this. In Newport it lies between the very- 

 ancient conglomerate that has been described, and the granite ; al- 

 though the granite does not appear, I believe, till we cross an arm of 

 the sea into Little Compton. 



Another aggregate, in this formation, which may be called quartz 

 rock, consists of red or reddish hyaline quartz, somewhat in grains, 

 with a small quantity of mica, so that the rock always has more or 

 less of a slaty structure ; though this is not always seen but by close 

 examination. (Nos. 309 to 318.) In some instances its layers become 

 quite thin, in consequence of an increase of the mica : but they are 

 still genuine strata ; for these layers have a schistose structure, not 

 coincident with their planes. This red quartz rock exceedingly re- 

 sembles red sandstone at a little distance : but when closely exam- 

 ined, it is found to consist chiefly of red hyaline quartz, which ex- 

 hibits little evidence of a mechanical mode of production. It passes 

 into a structure distinctly conglomerated, and may perhaps be the 

 equivalent of Dr. Macculloch's " primary red sandstone; " though I 

 have no evidence that it alternates with any primary rock : and it 

 contains no feldspar. 



Sometimes this rock abounds with veins of quartz giving it a very 

 rich appearance. I have noticed these veins 'most numerously in 

 the south part of Wrentham, near the place of explorations for coal. 

 (No. 318.) 



4. Talcose Aggregate, (Steachist 1 Phillips and Wood) (Nos. 320 

 to 323.) This is a slaty rock, composed of grains of quartz and 

 sometimes feldspar, with talc or steatite. It lies between the red 

 quartz rock and the primary rocks in Walpole ; though not, there- 

 fore, older than the quartz rock ; since the dip of both is such there, 

 as to bring the talcose rock uppermost. It often exhibits distinct 

 fragments of previous rocks, and passes into a breccia or conglomer- 

 ate ; as in a quarry in Cambridge, two miles west of the Univers- 

 ity. It is not abundant in the graywacke formation j though many 

 of the oldest varieties of this formation have a talcose appearance. 



5. Classical Graywacke. I mean by this term to designate the 

 rock described by Werner's ablest commentator, Professor Jamesoii. 

 He says that Graywacke " is composed of angular or other shaped 

 portions of quartz, feldspar, Lydian stone, and clay slate, connected 

 together by means of a basis or ground of the nature of clay slate, 

 which is often highly impregnated with silica, thus giving to the mass 



33 



