108 



GEOLOGY OF THE NARRAGANSETT BASIN. 



disjoined parts of a small dike of amphibolite, 1 probably altered diabase, 

 had, in 1887, the appearance shown in the accompanying diagram (fig. 4). 



The contact relations of the limestones with the schists which inclose 

 them have not been satisfactorily determined, by reason of the lack of good 

 exposures. So far as the evidence yet obtained goes, the limestones appear 

 to be of sedimentary origin, though no trace of fossils has been found in 

 them. The lack of continuity in the exposures is probably due to com- 

 pressive movements in the formation of a closely folded series. It is 

 probable that the ovoidal form of the outcrops as represented on maps 

 would have to be made elliptical, or even pointed at the ends, if better 

 exposures existed. 



On the hypothesis of close folding, which alone can account for the 



great breadth of outcrop of this 



series on the western side of the 

 Carboniferous basin, the thicker 

 limestones, if newer than the slates, 

 would fall in synclinal axes. 



In the bed of a small brook 

 above the old iron mine in Crans- 

 ton a vertical bed of limestone may 

 be seen between the beds of slate. 

 The limestone has a thickness of 

 about 10 feet. It here seems to be an intercalated bed. As yet no facts 

 have been discovered to show whether the limestones are of the same age as 

 or newer than the slates and schists. It is clear that some of the limestones 

 are not older than the argillaceous series. 



The most conspicuous feature of the limestone areas is the occasional 

 association with them of metalliferous deposits. The richest and most 



Fig. 4. — Exposure of disconnected dike in Lime 

 Rhode Island. 



ock quarr 



1 Amphibolite from Lincoln. This rock occurs in the southernmost limestone locality at Lincoln, 

 Rhode Island. Microscopic characters: Hornblende occurs in the slide, with extinction parallel to 

 shorter diagonal of rhombs. Chlorite is present in large quantities. Muscovite occurs in confusedly 

 arranged irregular laminae. Titanite occurs in cleavage forms, derived probably through leucoxene 

 from decomposition of titaniferous magnetite. Magnetite in some areas still exhibits a kernel of the 

 unaltered mineral. Apatite occurs partly or wholly inclosed by the titanite. The highly altered 

 condition of this rock renders its determination doubtful. Quartz occurs in small areas, where it is 

 probably of secondary origin. Notwithstanding the absence of the feldspar constituent, the nature 

 of the hornblende makes it probable that the rock, now an amphibolite, was originally a diabase, like 

 many other highly altered dikes in southern New England. 



Mr. Schrader reports other dikes near the western margin of the Blaekstone series in the valley 

 of the Moshassuck River. There are also in this field apophyses from the granitic rocks. 



