410 SIMPSON : STRATA AND DEPOSITION OF THE MILLSTONE GRITS. 
distances ; so that it is frequently difficult to get anything like 
corresponding sections from the two sides of one hill. 
To understand this apparent disorder requires some knowledge 
of the probable mode and conditions under which they were laid 
down. This I shall attempt to describe shortly. For the present 
I shall give a short description of the beds as they appear in 
our locality, merely premising this, that the period of the Millstone 
Grit deposition was in all probability preceded by important changes 
of elevation and depression, unequal and local, doubtless, but still 
sufficient to give a distinctive lithological character to the beds of 
the series ; these oscillating earth movements, were not in all proba- 
bility confined to the sub-aqueous areas, but affected also the 
adjacent land areas, elevating them into regions where the 
mechanical and chemical forces of disintegration and denudation 
bad more effect, giving more rapid stream and current action, with 
consequent ability to carry along coarser detritus and distribute it 
over more extended areas. 
The lowest beds of the series illustrate this unequal subsidence, 
as they are laid upon a most uneven bottom and are extremely 
variable in thickness, and in the relative distances below the more 
evenly laid Rough Rock. 
In the Survey Memoirs of the Yorkshire Coalfield, a diagram- 
matic section, extending from Ashover, in the south, via Chatsworth 
and Grindleford Bridge, on through Midhopstones and Marsden, 
west of Halifax and Huddersfield, and northward to Keighley and 
Skipton, admirably illustrates this uneven floor and explains the 
variability in thickness and overlap of members of the series. 
In the south, from Ashover to Chatsworth, we get a fairly even 
bottom and constant beds ; then there commences a gradual depres- 
sion with its maximum depth around Rood Hill, from whence north- 
ward there is a gentle rise to south of the district between Penistone 
and Marsden ; a fairly even floor with sinking tendency to west of 
Huddersfield and Halifax, from whence northward the depression 
deepens considerably towards Keighley and Skipton. 
It is probably, then, to these depressions and their gradual in- 
filling we owe the great variations in thickness of the series ; varia- 
