138 GEOLOGY OF THE NARRAGANSETT BASIN. 



is due to its having been covered by a remnant of the ice sheet whose mar- 

 ginal sand plains surround the field on the east, south, and west. The 

 Carboniferous beds rest here upon the granitite and dip off steeply to the 

 north, in the form of a closed and puckered syncline plunging eastward, 

 the cross section of which structure is as follows, beginning on the south: 



1. A small knob of hornblendic granitite. This passes by almost insensible grada- 

 tions into arkose. 



2. Alternating beds of arkose and grits, with vein quartz pebbles, occasional 

 nodules of the granitite, and shaly partings. 



3. Quartz-pebble conglomerate, with sandstone partings, the latter holding casts 

 and hollows of fossil trees Irom a few inches to more than a foot in diameter, often 

 closely pressed and forming a mere gash with an ocherous cellular layer. (The fossils 

 described by Crosby and Barton in 1880 were the casts of this locality.) 



4. Red and green slates, with sandy partings. 



5. Fine quartz-pebble conglomerate in eastern part of the lot; wanting in the 

 western section. 



6. Red and green slates, like 4 above, with flattened and cylindrical casts; and 

 small greenish chloritic kernels, due to metamorphism. 



7. Quartz-pebble conglomerate, with sandstone partings, containing casts and 

 molds of fossil trees; one cast 12 feet long and 12 by 18 inches in diameter. 



Whole section 250 to 300 feet in thickness. 



Northward the beds are concealed beneath a swamp. This section is, 

 on the whole, one of the most instructive basal sections in the Carbonifer- 

 ous field of Massachusetts, and may be taken as typical for the general 

 history of the beginning of sedimentation in the main basin. 



Absence of basai granitic conglomerates. — It is a noteworthy fact that while the entire 

 Carboniferous section, amounting probably to a thickness exceeding 12,000 

 feet, is mainly conglomeratic, there is in this northern half of the main 

 basin no widespread basal conglomerate such as we find exactly at the 

 base of many sections in the geological record. The general absence of 

 basal conglomerates and the presence of arkose along the border indicate 

 clearly the condition of the land surface from which the earliest sediments 

 were derived. The failure to produce pebbles of the original rock under 

 the first attack of denudation must evidently have been due to the deep 

 disintegration of the granitic terrane whence the sediments were derived 

 and the low grades of streams. By the disintegration of the feldspar and 

 the decay of the iron-bearing silicates of the igneous rock the crystalline 

 ingredients fell into the state of coarse sand, and in this form the superficial 

 portion of the granitite was removed to the area of deposition. It was only 



