EROSION. 31 
fountain, which has now been denuded to the 2,400-foot contour. 
Ixactly what portions of the Green Mountain Range remained above 
kater during Cambrian and during Ordovician time is still uncertain, 
'he recent discovery by the writer of beach pebbles of pre-Cambrian 
meiss, 2 and 5 feet in diameter, in the Cambrian conglomerate along 
she edges of the gneiss area in Ripton, Vt., shown in the northeast 
corner of the map, PL I. points strongly to the existence there of shore 
conditions in Lower Cambrian time, but there is no way of deter- 
mining how long such conditions lasted after the Cambrian trans- 
gression. In Pownal, and again in Dorset, the Ordovician schists of 
the Ta conic Range come very near the Green Mountain Range. In 
the town of Florida, on Hoosac Mountain, Cambro-Ordovician 
Schists, some of which represent those of the Taconic Range, cross 
the axis of the range. Whether the absence of the limestone from the 
range be explained by a horizontal transition to argillaceous sedi- 
ments or by such an unconformity as that shown by the sections in 
the western part between the Cambrian and Ordovician, preventing 
calcareous sedimentation, 6 the western part of that portion of the 
Green Mountain Range shown on the map was probably once cov- 
ered by granular and schistose materials like those, and as thick as 
those, which occur on its western flank and on the Taconic Range. 
As the surface of the Green Mountain 'Range is generally above the 
2,000-foot level there should be added to the tinted areas of the map 
representing the 2,000-2,500-foot levels just east of the granular rock 
boundary — i. e., to the western rim of the anticlinorium — an amount 
equal to the average thickness of the granular and schistose rocks (2) 
and the schistose or slaty rocks (4), which is 4,300 feet, in order to 
restore the original Ordovician surface. 
That great changes must have taken place will be apparent to any 
student of the subject in looking across from the pre-Cambrian gneiss 
area on Hoosac Mountain on the line of Section .4 to the Greylock 
schist mass, when he finds that he has to look up from the anticlinal 
sea bottom on the 2,400-foot contour to the mass of sediments making 
up the adjacent synclinorium, the vertical difference being 1,100 feet. 
Either the anticline Jias been denuded of a vast mass of sediments, 
or it was aland surface which has itself been greatly denuded, or the 
Greylock trough has been thrust up. Of these alternatives the first 
is the most probable. 
It appears, then, that the amount of denudation has been very much 
greater on the once submerged portions of the Green Mountain anti- 
cline than on the Taconic synclines, although this difference is not 
quite so marked when points on the Green Mountain Range in which 
«Mon. U. S. Geol. Survey, vol. 23. 1894. pi. 1. 
6 Professor Wolff regards this interpretation of the relations on Hoosac Mountain as 
inadmissible. 
