20 BULLETIN 05 THE 



shall hereafter note in the discussion of this district, one of the most 

 remarkable features in the Coal Measures is the entire absence of dike 

 materials in this wide area, a feature in which it is in sharp contrast with 

 all the neighboring portions of New England. The fact appears to be 

 that the dikes which intersect the rocks of Southern New England were 

 formed before the Carboniferous age, or if formed after for some reason 

 never penetrated the deposits of the Coal Measure series. 



Although the intrusive rocks of this area are a baffling element in 

 the effort to unravel its structure, the principal difficulty arises from the 

 drift coating which covers at least ninety-five per cent of the surface. 

 In passing over the country, the student is, on account of the distribu- 

 tion of this drift coating, led to give too great structural importance to 

 the conglomerates and to the hornblendic granites. The fact is, that 

 the softer shales almost always occupy the lowest parts of the area. 

 Almost all the stream beds course upon them, and it is only by a care- 

 ful study of the drift materials that the preponderance of these slates 

 becomes evident. 



Origin of Sediments, and Conditions of Deposition. 



The sediments composing this Cambrian section appear to have been 

 derived f-om rocks substantially the same as those which now lie in the 

 field west of the area. Although fossils have been found in a small part 

 of the section, close study makes it plain that by far the greater portion 

 of the strata are clearly azoic. The frequent return of conglomerate 

 layers and the coarseness of the pebbles show that during most of the 

 time when the beds were accumulating the region was near shore ; so, 

 too, the large amount of sandy matter even in the slates affords a pre- 

 sumption that the region was not remote from the coast line. About 

 one hundred feet of shale beds have been subjected to a very careful 

 search for organic remains. The total thickness of the deposits in which 

 any trace of life has been found probably does not exceed one hundred 

 feet, and even in this section only a small part of the rocks actually con- 

 tain fossils. As before remarked the rocks of this Cambrian series are 

 very little metamorphosed. "VVe therefore cannot attribute the absence of 

 life to secondary changes, but must regard it as an original characteris- 

 tic of these sediments. The great abundance of conglomerates, the 

 considerable size of their pebbles, the fact that none of these have a 

 beach-worn character, but are in general form like the pebbles con- 

 tained in the neighboring glacial deposits of a stratified character, afford 



