u} 
MANSFIELD: ROXBURY CONGLOMERATE. 259 
to be younger than the folding. The overthrust fault at Chestnut 
Hill Reservoir belongs to a different type and probably accompanied 
the folding. The east-west block faults may, however, have accom- 
panied the deposition of the strata. 
According to this view the deposition of the sediments, begun during 
a broad differential uplift of the region, may have become more or 
less localized in orogenic basins developed by block faulting during 
later deposition. ‘The main blocks thus formed, so far as this region 
was concerned, were perhaps the northwest highland region and the 
Supposed upland to the southeast, now no longer extant. Between 
lay, perhaps, several minor blocks, one of which, upthrown, may now 
be represented by the Blue Hill Range. As deposition progressed, 
doubtless the minor blocks may have been buried by the accumulat- 
ing sediment from the higher lands; but there is no definite evidence 
that at any time the respective basins were without outlets. 
How extensive the region thus affected may have been there is no 
means of knowing. The occurrence of the conglomerate areas at 
Bellingham, Woonsocket, and Harvard and in the basin of the Par- 
ker river suggests that the conglomerate may formerly have widely 
exceeded its present limits. Faults of the block type occur fre- 
quently in the crystalline rocks of the region, especially in the north- 
west highlands; but in rocks of such similar character the amount 
of displacement is not easily estimated. The crystalline floor upon 
which the sediments may have been deposited has been carried 
above the plane of erosion and the overlying strata have been worn 
i away. 
GENERAL SUMMARY.— (1) The estimated bulk of sediments now 
remaining in the three basins, assuming the dimensions given and 
an average thickness equal to half the maximum figures, is approxi- 
mately 1,700 cubic miles. 
(2) An attempt to restore the present beds to their previous unde- 
formed condition shows that they would form a mass at least 60 miles 
long and 30 miles wide, attaining a height above their base of 12,000 
feet toward the south but thinning down to 5,000 feet or less northward. 
(3) Prior to the deposition of the strata the present basins were not 
certainly outlined. 
(4) The arkose and red beds indicate that the region was subjected 
to long subaérial decay in Precarboniferous time under conditions of 
‘Cool climate and moderate or scanty rainfall. 
(5) The region at that time was probably bounded on the east and 
