146 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
dissect it. In ascending the upper member of the fold, however, we 
first passed up a fairly mature ravine and then up a dividing ridge, 
because there was little or none of the original surface left. If this 
striking contrast in the degree of dissection prevails elsewhere along 
the double monocline it is a strong indication that after the fold was 
developed the upper part was subjected to erosion while the lower 
portion was yet concealed below the overlying baselevelled Permian. 
Later stripping of the Permian revealed the lower member of the fold, 
and exposed it to erosion in the next cycle. 
Some miles south of House Rock Ranch, at a point where the mono- 
cline rapidly steepens its dip with an accompanying westward swinging 
of the base line, there is found a series of well marked terraces. These 
terraces bevel across the softer beds which overlie the Carboniferous 
limestones in the plateau series, slope gently downward toward the 
east, and are capped by a considerable thickness of alluvium consisting 
largely of limestone debris. They are now much dissected, but still 
persist over a considerable area, marking two or three distinct levels; 
the lowest series extends far out across the platform east of the mono- 
cline, and an unknown distance to the south. The higher terraces 
cover a much more restricted area than the lower series. A small 
remnant of a similar terrace is found near the foot of the monocline 
just north of the ranch. 
That these terraces mark a baselevel of erosion there can be no doubt. 
But it is cjuite possible that they represent a temporary local baselevel, 
not of great significance in the general history of the region. Their 
greatest significance lies in their relation to the monocline. According 
to the antecedent theory, the softer beds overlying the limestone were 
largely removed from the ]Marble Platform region before the folding, 
although occasional remnants remained. The preservation of these 
remnants could bear no relation to the great monoclinal fold, believed 
to be developed only after this work of erosion was largely completed. 
According to the alternative theory, the fold was developed at an 
earlier period and included the overlying rocks in the up-arching. 
Subsequent erosion to general or local baselevels would bevel the 
softer beds east of the monocline and the bevelled surface might be 
covered by debris brought down from the hard limestone arch which 
Avas too resistant to experience rapid reduction. Later erosion would 
dissect the debris-covered surface, leaving remnants as terraces. 
These terraces would remain longest near the base of the monocline. 
