CLARK: TACONIC REVOLUTION. 149 
evidence from this portion of North America. Where known 
Ordovician rocks do occur, they either are not seen in contact 
with the overlying strata or else they appear to be a part of a 
conformable series including at least the Silurian. After a close 
study of the descriptions of the sections presented above the 
author is led to offer a suggestion which, to be sure, cannot be 
given much weight at present, namely, that the so-called Cam- 
bro-Silurian and the known Silurian and early Devonian strata 
may have been deposited in a conformable, or nearly so, series, 
with no greater discordance in dips than is recorded from the 
undisturbed rocks of New York State. After the Devonian 
sediments were deposited, folding of the whole series may have 
taken place, in which the upper and more resistant (competent) 
beds resisted the stresses to a greater degree than the older, non- 
competent shales.^ These latter would be more likely to become 
crumpled and plicated, not only because of their lack of com- 
petence but also because of their position below hundreds and 
probably thousands of feet of superimposed strata. If the 
stresses were severe enough, thrust planes might well have been 
developed, and the most likely place for these would have been 
along the boundary between the lower shales and the upper 
hmestones and sandstones, more or less parallel to the stratifica- 
tion. If a thrust were developed here, it is obvious that further 
crumpling would befall the shales as the heavy load moved over 
them. The over-riding rocks, not being themselves burdened 
with an overpowering load of superincumbent rocks, and pos- 
sessing some high degree of competency, would not be likely to 
suffer more than gentle folding. The thrust plane, like all con- 
tacts in this country, would probably be obscured, and a casual 
survey of the field evidence would lead the observer to suppose 
that it pointed toward an erosional unconformity — a series of 
highly plicated shales, capped by a series of gently folded more 
massive rocks. 
At present, this suggestion is incapable of proof, but we may 
look forward to detailed work in a few restricted areas to settle 
the point one way or the other. Just now we must confess that 
the state of our knowledge does not permit us to draw con- 
'See Bailey, L. W., Trans. Roy. Soc. Caaada 1909, ser. 3, vol. 3, sect 
4. p. 55, 1910. 
