42 TJie American Geologist. juiy, isos 
niorpliism. The feldspar cannot have lieen carried very far, other- 
wise it would have been destroyed. 
There is evidence that the pre-Cambrian surface was an eroded 
land surface. The chief evidence of this is found in the Stam- 
ford dike of Clarksburg mountain, near Williamstown, which was, 
in pre-Cambrian times, eroded out below the general level of the 
surface. Into this the over-lying Cambrian sags, and it contains 
fragments of weathered dike rock. 
Professor Pumpelly suggests that when the Cambrian sea en- 
croached on the land the country was covered by a deep secularly 
decayed- covering. The residual soil and all the finer materials 
were quickly removed to a distance ; but the deeper zone of semi- 
kaolinized material was only partly worked over and was deposited 
near at hand. Below this was the semi-disintegrated zone now 
represented by the transition beds of highly foliated granitoid 
gneiss. This, by its weakened condition, during the subsequent 
extensive folding, behaved like the worked over material above, 
and is therefore laminated. Hence the transition and apparent 
conformity between the schist and granitoid gneiss. It will be 
seen that this is a highly plausible theory which explains, so far 
as is at present known, all the peculiarities of this most puzzling 
region ; and it seems to be the only theory which will exj)lain the 
facts found in the field. 
These facts lead professor Pumpelly to suggest that most of 
the liasal beds of sandstone, conglomei*ate and quartzite, marking 
the beginnings of geological periods, are produced in this way, 
iSimilar beds occur on the eastern edge of the Adirondacks where 
the feldspar pebbles, as in the case of the Hoosac mountains, 
have not Ijeen divested of their semi-disintegrated shell. 
Pre-Potsdam secular decay has been descrilied liy Newton from 
the Black Hills, by Peale in the pre-Cambrian granite of the South 
Platte region in Colorado, and by ethers from various parts of 
the west. Pumpelly* has suggested a pre-Silurian disintegration 
of Pilot Knob, Missouri, and later explorations l)y mining have 
verified this suggestion })y finding a deep zone of disintegration 
on the Silurian. In all these cases the transgression of the sea 
has in part worked over the materials of disintegration and buried 
the lower parts in large measure, beneath the sediment thus de- 
*Geol. Survey Missouri, 1873, i, 12. 
