150 LATERAL PRESSURE IN THE BRISTOL DISTRICT. 
amount is not inconsiderable. If, for example, we assume that 
the Clifton fault is the only serious dislocation of the strata 
between Penpole Point and the margin of the coalheld beneath 
Bath, a distance of about twelve miles, this one fault gives 
evidence of a diminution of the whole width of I'd per cent., 
or nearly eighty-four feet in the mile. 
The diminution due to folding — or the arching and trough- 
ing of the strata under lateral pressure — admits of somewhat 
more accurate determination. Let us take some selected 
plane for detailed consideration. That which I have chosen 
for the estimate of the effects of folding in the Bristol district 
is the plane or the base of the Lower Limestone Shales, or that 
dividing the Carboniferous system from the Old Red Sand- 
stone. Recorded observations and measurements give us the 
thickness of the overlying series within the Carboniferous 
system. The dips are known or ascertainable, and the pub- 
lislied maps of William Sanders and the Geological Survey 
give with sufFicent accuracy the boundaries of the constituent 
series. 
Taking these boimdaries as reference lines the depth of the 
selected plane below sea-level, in the synchnes, and the height 
above the sea-level at which this plane would occur in the 
antichnes, if the strata had not been partially removed by 
denudation, can be readily calculated. The curved hne 
representing a section of this plane — ^horizontal and vertical 
distances being represented on the same scale — may be drawn 
by interpolation between the points directly calculated. The 
total length of the curved plane between any assigned hmits 
can thus be measured, and it gives the originally horizontal 
extent of the plane. The existing horizontal distance between 
the hmits gives the approximation of these limits during 
compression, and a comparison of the two gives the percentage 
diminution of horizontal extent due to lateral pressure. 
It is unnecessary to give the details of the sections I have 
thus drawn. In a section between Tortworth Court to the 
