Flinty Slate and Chert. 



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granite : but where a certain proportion of argillaceous and siliceous 

 matter is contained in it, the effect of heat will be to render it more 

 hard and compact. 



A portion of the graywacke slate near Fort Adams is converted 

 into jasper. But a more particular description must be deferred till 

 I have finished what I have to say concerning flinty slate and chert. 

 I shall also have occasion to speak again of the striking evidence, 

 which the group of rocks above described in Newport, furnishes, of 

 the igneous origin of granite. 



Flinty Slate of Nahant. 



The greater part of this promontory is sienite. But enough of 

 the argillaceous slate remains at its southeastern extremity, to show 

 the geologist the influence of trap veins in passing through it. 

 These are quite numerous, both in the slate and in the sienite ; and 

 sometimes the greenstone is intruded laterally between the strata of 

 slate, in the form of beds. Yet the general dip and direction of the 

 slate appear to be but little affected by these veins, although they fre- 

 quently constitute more than half the rock. For the basset edges of 

 the slate run nearly east and west, and dip northerly; which cor- 

 responds with the general direction and dip of the argillaceous slate 

 in that region. Nearly all the slate, however, on this promontory is 

 much indurated ; and a considerable proportion of it converted into 

 genuine flinty slate. The slaty structure is rarely lost, except at the 

 junction of the greenstone and slate, where the two rocks are so in- 

 timately blended, that it is not easy to fix upon the spot where either 

 of them commences. This corresponds with the opinion of Dr. 

 Macculloch, that nothing but the requisite degree of heat is necessary 

 to convert argillaceous slate into greenstone. (Nos. 383 to 385.) 



Chert. 



I have already described a conatus for the production of chert 

 from the Newport limestone. But at Nahant the process seems in 

 some cases to be nearly or quite completed. One observes there, that 

 a considerable proportion of the flinty slate contains layers of a light 

 gray substance, somewhat resembling in aspect and fracture, certain 

 varieties of pottery. (No. 386.) On examination we find intermixed 

 with this substance, a compact or even semi-crystalline limestone. 

 In short, we observe every degree of induration and compactness 

 from limestone to chert. There can be little doubt, it seems to me, 



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