EPICONTINENTAL SEA OF JURASSIC AGE 261 
two hundred feet. In fourteen localities the thickness is under 
four hundred feet. These localities are scattered throughout the 
length and breadth of the interior province. In all the areas for 
which greater thicknesses have been recorded there are none in 
which the entire thickness could, without question, be assigned 
to the Jura. 
The lithological character of the beds is much the same for 
all areas. The formation consists everywhere of essentially the 
same group of arenaceous clays, shaly marls, impure limestones 
and sandstones. The order of succession of the beds implies 
ever changing conditions of sedimentation. Thin beds of sand- 
stone are overlain by thin beds of fossilferous clays, marls, or 
limestones; and these in turn are followed by another similar 
group. 
The absence of any considerable thickness of limestone over 
a large area indicates that for no great period of time were the 
waters of the sea entirely free from clastic sediments. The 
presence of cross-bedded sandstone and ripple-marked layers at 
different horizons, the almost universal presence of Ostrea and 
other shallow water forms, together with the stratigraphic and 
lithologic characters just mentioned prove that the waters of the 
sea were not of great depth; that the sea was not of the abysmal 
type. It was not a sea comparable in depth to the Mediterranean 
but was a shallow epicontinental sea. From the geographic dis- 
tribution of the known Jurassic the outlines of this sea were as 
indicated on the map’ accompanying this paper. 
From the character and extent of the sea it may be assumed 
that no extensive epeirogenic movement was necessary for its 
inauguration, providing the antecedent topographic conditions 
were favorable. Inthe northern part of the area there is evidence 
that a considerable period of erosion preceded the Jura, as the 
Red Beds are absent and the Jura rests on the Carboniferous. 
This period of erosion may have been sufficient to reduce the 
land area to approximate base level in which case a very slight 
warping would have been sufficient to let the waters of this 
«See p. 245. 
