MUSEUM OF COMPAKATIVE ZOOLOGY. 15 



up this field at an earlier date. lu 1884, my service as Geologist in the 

 United States Geological Survey enabled me to command more time for 

 these explorations, and the rapid advance of the topographic work in 

 this area done by that Survey in co-operation with the State of Massa- 

 chusetts has afforded a sufficient topographic basis for the inquiry. 



I am indebted to my assistant, Mr. August F. Foerste, for a certain 

 amount of aid in the preparation of this report. He has worked out a 

 part of the boundaries which arc delineated in the accompanying map, 

 and has collaborated with me in the preparation of the descriptions 

 contained in the second part of this I'eport. 



General Structure of the Field. 



The area indicated in the first of the accompanying sketch maps 

 evidently contains two distinct series of stratified rocks, besides the 

 numerous and peculiar injected materials, which are not to be discussed 

 here. On the east we have the Coal Measures of the Narragansett 

 basin. The western boundary of this series cannot be exactly traced, 

 fur the reason that it is altogether hidden by drift deposits mostly 

 belonging to the class of kame and terrace accumulations, and therefore 

 unfit to afford the basis of any determinations as to the subjacent beds. 

 West of this boi'dor, the position of which cannot be at any point fixed 

 within the limits of some hundreds of feet, we have the area of Cam- 

 brian deposits. This strip has an average width of not far from two 

 miles. Although its contact with the Carboniferous deposits is not 

 seen, it is likely that it belongs to the class of erosion overlies, that 

 is to say, the Carboniferous rests upon the worn surface of the steeply 

 inclined Cambrian beds. Evidence in favor of this supposition is also 

 afforded by the conglomerates of the Coal Measures, which contain 

 more or less detrital material brought from the Cambrian series, which 

 was evidently exposed to erosion at the time when the lower portion 

 of the Coal Measure deposits were formed. Moreover, as my exten- 

 sive studies on this district have adequately shown, few faults occur 

 in the field. The habit of accident is that of folding rather than 

 faulting. 



On the west of the Cambrian lies another field of rocks, which I am 

 compelled at present to consider as of Pre-Cambrian age. The deposits 

 in this section consist in the main of gneissoid rocks of varying compo- 

 sition and a great area of dark greenish chloritic deposits which appear 

 in part at least to be metamorphosed conglomerates and shales. In the 



