418 SIMPSON : STRATA AND DEPOSITION OF THE MILLSTONE GRITS. 
variable thickness, and for a certain space is entirely absent, the 
Lower Coal Measures overlapping westward on to the limestone. 
This upheaval was of a general nature, affecting a considerable 
region, giving the rivers a greater velocity, and a capacity for carrying 
coarse sand much greater distances. 
Further evidence is, I think, furnished by the change in the life 
of the Grit series. Whilst the Carboniferous Limestone and the 
Yoredales were undoubtedly deposited in marine areas,the paleontology 
of the Grit series is that of brackish water, estnarine, or lagoon type, 
with only occasional marine horizons. The upheaval was probably 
responsible for shutting off a great shallow water basin from com- 
munication with the outer ocean, interposing barriers which it would 
appear were still occasionally broken down. That this movement of 
upheaval gradually exhausted itself prior to the deposition of the 
Coal Measures is evidenced in the gradational changes in the texture 
of the sandstones from the Millstone Grit upwards. 
The Grits are, in the most important beds, not only coarse and 
massive, but show considerable persistency over large areas, whilst 
the Coal Measure sandstones are generally much finer grained, more 
variable in thickness, and less persistent, and were evidently deposited 
in a more regularly subsiding area. 
The shallow water conditions that favour accumulation of sand- 
stones must have been of longer duration in the Grit period, and the 
shoaling necessary for coal growth more rare, and for the most part 
local, and of comparatively short duration ; in other words, the period 
was mainly one of steady subsidence of the sub-aqueous area, the 
pauses being few and far between. 
The Middle Grits however have much in common with the Coal 
Measure type, and show 7 frequent traces of littoral conditions in ripple 
markings, worm trails and casts, rain-prints, etc., and evidence similar 
delta, lagoon, and swamp conditions. A shallow water area, receiving 
supplies of mechauical sediment from various sources. At times por- 
tions shoaled, and the streams and currents cut changeable channels 
in all directions in these shoal banks, channels that on furthur sub- 
sidence, perhaps received deposits of different texture, and so give us 
confusing abutments of sandstone and shale, and fault-like sections, 
where the beds above and below are unbroken. 
