MUSEUM OF COMPARATIVE ZOOLOGY. 15 
up this field at an earlier date. In 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- 
chnsetts 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 are delineated in the accompanying map, 
and has collaborated with me in the preparation of the descriptions 
contained in the second part of this report. 
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, 
for 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 border, 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 tc 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 
