BLAKE : EAST YORKSHIRE. 
19 
clays and ironstones of the upper part of the Lower Lias in 
such a manner as to deceive by their similarity the working 
but uninstructed miner of our day. The absence of bitumen 
in these clays may be accounted for by the probability of the 
interval between the two coal fields not being very produc- 
tive. At last the coal measures were exhausted, and the 
millstone grit began to be attacked. There was less of this 
to wear away, so the results were smaller; but they were 
sandstones like the originals, and are seen in beds of that 
character in the Middle Lias. These once more gave place to 
the underlying rocks as a main source of deposit, and the 
Yoredales and Scar limestones yielded the increasingly cal- 
careous rocks with which the Middle Lias terminated. 
Although these are now ironstones, there cannot be much 
doubt as to their original nature, and that they have under- 
gone subsequent change. "Whence the iron came that took 
the place of calcareous matter may not be clear, though 
possibly the basalt which formed the wider extension of the 
whin-sill may have contributed some. 
The great oysters of this period, and other associated 
fossils, indicate a shallowness of the water, owing to the rate 
of deposition being greater than that of the depression ; but at 
last a change came, the neighbouring land was depressed 
with the sea, a wider drainage area took in some of the coal 
measures once more, and gave us the shales of the Upper Lias, 
with their jet-forming bitumen derived from the richer beds 
of coal. This Upper Lias has a less thickness than the Lower, 
because it was derived from a smaller area of coal measures, 
and it more quickly filled the sea. Thus, by the time the 
millstone grit and lower beds again became the source of 
supply, the sea was so shallow that nothing but estuarine 
beds could be formed, into which the plants of the Oolitic 
period were carried. Thus, it is to local circumstances that 
the estuarine character of the Yorkshire Inferior Oolite is due. 
