10

Formation. Thickness.
A. Upper shales and sandstones.........-..+.2-+++ee- 1,500 feet.
B. Coarse conglomerates... ..... 06.0.2 6.02 eee ee ees 25000:5<*
C. Lower shales with coal and iron ore...........0....- S000. *
Dz. Agslomerates.. 225 652 5085 gS + ee ees 33500;.*
. SEwer SANUSEOUCS. 04. ne one eae ae bee soe Se 1,900 7°
T5000: «*

This subdivision is not entirely correct, as there is a marked
unconformity, structural and paleontological, between sub-
division C and subdivision D. A, B, and C are Upper Cretaceous
in age, D and E are middle and lower Jurassic respectively.
That such a minute observer as Dawson considered this uncon-
formity ‘‘essentially unimportant” is largely due to the fact that
the Cretaceous and pre-Cretaceous beds are in many instances
lithologically similar. This is so much the case, that only by the
closest attention to the stratigraphy and structure, and by care-
ful labelling of each collection of fossils, was the present writer
able to separate them. Dawson, on structural and lithologic
grounds, mapped several areas of pre-Cretaceous rocks as
Cretaceous. The fossils collected from beds that he supposed
to be wholly Cretaceous, on examination proved to be partly
referable to the Upper Cretaceous, and some species to the
Jurassic. This condition of affairs caused uncertainty in regard
to the age of these rocks, which were, until the present examin-
ation, supposed to be Lower Cretaceous.

It has been mentioned that Dawson considered the uncon-
formity between subdivisions C and D to be “essentially unim-
portant.” His conclusion was largely based on an examination
of the rocks of Alliford bay, which are shaly and tufaceous sand-
stones, quite similar to those occurring in the Cretaceous. On
the east side of the bay an apparent transition between true
volcanic tuffs and sandstones occurs, and is figured by Dawson
on page 68B of his report. The writer made a careful study of
this locality, and fortunately found abundant fossils in the
rocks of Alliford bay, which prove them to be wholly Jurassic.
Dawson was right, therefore, in saying this unconformity was
unimportant, but he did not recognize the much greater and more