SHALLOWNESS OF THE MESOZOIC SEA. 
211 
of the signs of very shallow water with the presence of decisive signs of 
it in the Mesozoic and Permian, that we are drawn to the inference of some- 
what greater marine depths in the early and middle Carboniferous. 
In the upper Aubrey series we come upon some indications of shallow 
water, and from the base of the Permian upwards these are ever present. 
In the Permian, Trias, and Jura we find instances of those peculiar uncon- 
formities by erosion without any unconformity of dip in the beds. Perhaps 
the most widely spread occurrence of this kind is the contact of the sum- 
mit of the Permian with the Shinarump conglomerate which forms the base 
of the Trias. Wherever this horizon is exposed this unconformity is gener- 
ally manifest. Between the base of the Permian and the summit of the 
Carboniferous a similar relation has been observed in numerous localities, 
and there is a similar instance in the lower Trias. It has also been detected 
between the Trias and Jura, and between the Jura and Cretaceous. We 
are tempted to ask here, whether such unconformities, without the slightest 
trace of permanent displacement in the strata, may not have been due to 
oscillations in the regional sea-level rather than to movements of the 
land? 
One of the more striking features of the lower Trias is the occurrence 
of a vast abundance of silicified wood. It is not uncommon to find large 
tree trunks imbedded in these shales in good preservation. They are also 
found in a fragmental condition among the pebbles of the Shinarumjj con- 
glomerate. These petrifactions are found over a wide extent of country 
from the Sheavwits Plateau along the front of the Vermilion Cliffs to the 
Paria, and again far to the northward at the base of Thousand Lake 
Mountain in the district of the High Plateaus. 
These occurrences and others, which will soon be specified, point de- 
cisively to the inference that during the great era of accumulation, lasting 
from the closing stages of the Carboniferous to the Eocene, the surface of 
deposition never varied far from sea-level, and now and then the waters 
retreated from it, but only for very brief periods. On the whole the depo- 
sition proceeded almost continuously. It necessarily follows that in the 
long run the underlying beds sank deeper and deeper as the newer ones 
were piled upon them. This fact is but a repetition of what is found in 
other regions where the deposition has been very heavy. The strata sub- 
