258 



Scientific Geology. 



a considerable degree of hardness. The imbedded portions vary in 

 size, but seldom exceed a few inches in breadth and thickness." Bro- 

 chant does not include in the term graywacke any variety of rock 

 " whose grains exceed the size of a hazle nut." Hence the conglom- 

 erates that have been described above, cannot be regarded as classical 

 graywacke. This is the opinion of Professor Webster ;* whose op- 

 portunities for a personal examination of European graywacke, give 

 his decision on this point great weight. But associated with these 

 conglomerates, we have rocks of a much finer grain, whose compo- 

 sition corresponds essentially with the above definition ; (Nos. 324 to 

 334,) although every ingredient may not in all cases be present. 

 Sometimes the mass is colored red by the presence of the red oxide 

 of iron ; as in Attleborough. But more commonly it is gray, as in 

 Rehoboth. It often becomes fine grained and passes into graywacke 

 slate, as at the quarries in Pawtucket ; where it is traversed by nu- 

 merous veins of quartz mixed with calcareous spar. 



6. Graywacke Slate. This variety of rock is quite common in 

 this formation. Its colors are either gray or red; and it- appears 

 to be composed in a great measure of wacke. Mica, however, some- 

 times enters into its composition. Its structure is always slaty: but 

 the layers are much more irregular and tortuous than argillaceous 

 slate, and its aspect more earthy ; though it is no easy matter to draw 

 a line between them. It is traversed frequently by veins of quartz. 

 (Nos. 335 to 346.) 



7. Argillaceous Slate. The argillaceous slate in the eastern part 

 of the State is so intimately connected with the varieties of rock 

 above noticed, that it ought in justice to be described as one of the 

 members of the graywacke group ; although marked as a distinct 

 deposite on the map. That this is one of the oldest varieties of this 

 group, I have no doubt ; but certainly not older than some that have 

 been mentioned. I am aware that fragments of this slate occur in one 

 of the varieties of conglomerate that have been described ; and this 

 not only shows the posterior production of the latter, but renders it 

 doubtful whether both rocks were produced during the same geolog- 

 ical epoch. But with the knowledge that I possess of this series of 

 rocks, I fear that an attempt to divide them would only introduce 

 confusion into my account. I would not pretend to a degree of ac- 

 curacy to which I have not attained. 



* Boston Journal of Philosophy and the Arts, Vol. 1. p. 289. 



