— u. 
MANSFIELD: ROXBURY CONGLOMERATE. 239 
The dimensions of the various basins as given in preceding pages 
are too indefinite to be of absolute value but they serve to give a general 
idea of the areal content of these regions. ‘The average thickness 
of the sediment in the several basins has not been determined but it 
may be fair to assume that it is equal to at least half the maximum 
amount indicated by the figures just given. Assuming the area of 
the Boston Basin'to be 125 square miles (see page 98), and the average 
thickness of the sediments, including the slates, to be 2,650 feet, or 
roughly half a mile, the bulk of the Carboniferous rocks included 
may be estimated at 62.5 cubic miles. Similarly, if the Norfolk Basin 
is assumed to be 20 miles long and to have an average width of 1.5 
miles (see page 220), the area will be 30 square miles, and if the aver- 
age thickness is assumed to be 3,750 feet, or approximately 0.7 miles, 
the bulk of the Norfolk Basin sediments may be estimated at 21 
cubic miles. According to the figures given by Shaler (¢., page 231), 
the Narragansett Basin is approximately 50 miles long and 30 miles 
wide, giving an area of 1,500 square miles. Assuming the average 
thickness to be 6,000 feet, or about 1.1 miles, the bulk of the Narra- 
gansett sediments is 1,650 cubic miles.* The total bulk of the Carbon- 
iferous rocks now preserved in the three basins, according to these 
figures, is 1,713.5, or roughly 1,700 cubic miles. When it is remem- 
bered that vast periods of erosion have intervened since the deposition 
of these rocks, and that an unknown but probably great proportion 
of the original mass has been thus swept away, it will be seen that 
the former land area which furnished the Carboniferous accumula- 
tions must have suffered intense erosion, during this deposition, and 
must have undergone important changes of form. 
Local Character. The materials, of which this great sedimentary 
Mass is composed, aré mainly the same as the rocks now exposed 
around the borders of the basins. Noteworthy exceptions are the 
fossiliferous quartzite and muscovite-granite pebbles found in the 
Conglomerates of the Narragansett Basin, to which reference has al- 
ready been made, (page 164). 
_ The Problem. The problem now to be considered may be stated 
Interrogatively as follows:-- What were the agencies, by which so great 
a bulk of local materials was removed from its parent rock and accu- 
mulated in the present areas, and under what conditions were the 
deposits made? 
GEOGRAPHICAL CONDITIONS.— Restoration of Strata. One of the 
_ | This estimate is slightly too large since the Norfolk Basin has inadvertently been 
Included and is therefore counted twice. The final estimate is not materially affected. 
