100 GEOLOGY OF THE NARRAGANSETT BASIN. 



as to produce excessive complication of structure on the one hand and 

 to remove great thicknesses of strata on the other, the evidence which 

 the geologist seeks for the reconstruction of ancient deposits may be too 

 scanty to permit him to obtain satisfactory results. The stratified rocks of 

 the Narragansett Basin long since ceased to lie in their original attitudes, 

 and so much of them has been carried away by erosion that their strati- 

 graphic succession can be made out only with difficulty and only for limited 

 portions of the field. But these general causes, which, by reason of their 

 long-continued action or' their intensity, have worked to the detriment of 

 geological investigation in most mountain-built districts, have in this field 

 been reenforced by local peculiarities arising from the geographical position 

 of the area and from events of recent geological occurrence. 



The difficulties encountered in making out a complete and satisfactory 

 succession of the strata of this field may be stated as follows : 



REPETITION OP LITHOLOGICAL CHARACTERS. 



The duplication in texture and color of sediments widely separated 

 chronologically, but in close juxtaposition, either by superposition through 

 unconformable deposition or by folding and faulting, is a source of doubt 

 where fossils are not present in both terranes. Thus, in North Attleboro, 

 Massachusetts, and northward, in the midst of an area occupied by red 

 Carboniferous shales, sandstones, and conglomerates, there appear red 

 Cambrian shales not to be differentiated in most localities except by means 

 of the contained fossils. Until Cambrian fossils were discovered, the red 

 Cambrian strata were included by all observers with the red series of more 

 recent date. 



TRANSITION OP LITHOLOGICAL CHARACTERS. 



By gradation in the size of the particles in a stratum, a conglomerate 

 on one side of a denuded anticline or syncline may be represented by a 

 sandstone or shale on the other side of the same broad fold. In like man- 

 ner, the coloration of beds may vary from one part to another of the same 

 basin, so that strata are no longer distinguishable. In the northwestern 

 part of the Narragansett Basin there are thick beds of red color, having a 

 fairly well defined stratigraphic position. Farther south these beds are 

 replaced by others of dark-gray color, and are even underlain by kinds 

 of beds which in the northern area alwavs overlie the red series. 



