LOCAL OOFDITIOIsrS AFFECTIIiTG OBSBEYATIOK 103 



between it and the band of rock next north. Where the strata strike in a 

 northerly direction, the fanning out of the drift southward makes the deter- 

 mination of less value; but trains of bowlders thus formed are frequently 

 of great use in checking observations upon isolated outcrops. In using 

 this method less reliance is to be placed upon waterworn diiffc than upon 

 ice-laid deposits. 



Recent depression of the land has, in the immediate vicinity of 

 Narragansett Bay, caused the flooding of old valleys, so as to isolate rock 

 areas in the form of islands and to conceal strata which might otherwise be 

 open to examination. To a certain extent this difficulty is compensated by 

 the good exposures in the cliffs formed along the seashore by the action of 

 waves.* Such natural sections occur around most of the islands in the bay 

 and along the coast where hard rocks come to the surface, as far north as 

 Providence. 



ABSENCE OF ARTIFIC^IAL EXCAVATIONS. 



The almost complete absence of artificial openings, either mines or 

 quarries, in this field at the present day has limited the observations here 

 detailed almost entirely to surface exposures. The quarries which exist 

 are mostly in the granitic rocks bordering the basin, and it is only from the 

 recorded observations of previous workers that information regarding coal 

 mines can be obtained. 



It is only by a recognition of these difficulties and their combinations 

 at various localities in this field that the geologist realizes the checks which 

 it is necessary to apply to his work in all stages of its advancement 

 These difficulties, along with a formerly prevailing misapprehension as to 

 the nature of secondary structure in rocks, are responsible for the general 

 belief among geologists that the strata of this area are too much broken to 

 be unraveled. On taking some of the earlier-drawn sections of the strata 

 of this basin into the field, it will be seen that over considerable areas 

 cleavage was mistaken for sti*atification,^ metamoiphosed Carboniferous 

 beds were taken for schists of much earlier date, the dips of strata were 

 averaged where they should have been analyzed and separately repre- 

 sented, and more reliance was placed on identity of color than this feature 

 is worth in determining the identity of origin of sediments. 



