Graywacke Conglomerates.. 



253 



posed of elongated rounded nodules of quartz rock, and quartz rock 

 passing into mica slate, with a cement of talcose slate. The nodules 

 vary from the size of a pigeon's egg, to four, and even six feet in their 

 longest diameter, and constitute the great mass of the rock. They 

 are so arranged that their longest diameters are uniformly parallel to 

 one another : lying in a north and south direction : which corres- 

 ponds with the layers of the schistose cement, and also with the gen- 

 eral direction of the strata in the vicinity. Both the nodules and the 

 cement abound in small, distinct, octahedral crystals of magnetic iron 

 ore. 



Amphibolic Aggregate. 

 Graywacke Slate. * 



The above rough sketch of the southeast point of Rhode Island, 

 will assist in rendering intelligible the relative position of ( this con- 

 glomerate, and also of three or four other varieties of this formation to 

 be hereafter described. About a quarter of a mile from the coast, three 

 precipitous bluffs, a, b, c, several rods wide, separated by salt marshes 

 from 15 to 20 rods wide, rise one or two hundred feet, trending north- 

 erly, and converging ; so as apparently to unite at no great distance. 

 The two most easterly ridges are very steep, and exhibit evidence of 

 having been powerfully abraded. The outer ridges, a, c, consist of 

 the peculiar conglomerate above described: the central one consists of a 

 hard graywacke slate, and a very singular and puzzling rock, which 

 I shall venture to describe as an amphibolic aggregate. Half a mile 

 southeast is an aggregate of quartz and mica to be described in the 

 sequel. 



