228 
POPULAR SCIENCE REVIEW. 
And since then the order of the older underlying rocks has 
been thus determined by Sir R. Murchison, Professor Sedgwick, 
and others : — 
1 
p3 
'Permian 
( Coal Measures 
Carboniferous - Millstone Grit . 
1 Mountain Limestone 
Devonian (Old Red Sandstone) 
Silurian ...... 
. Cambrian 
Gloucester, W arwickshire 
Somerset, Gloucester 
Somerset, Gloucester 
Somerset, Gloucester 
Devon, Somerset 
Wales 
Wales 
It was for a time supposed that secondary strata maintained 
in the main their regular sequence and thickness unimpaired 
over large areas, and in our early geological works the section 
of the secondary formations, from the west to the east of 
England, is given as under : — 
a. Tertiary strata. I c. Lower Greensand. I e. Lias and Trias. 
b. Chalk. I d. Jurassic series. | f. Palseozoic rocks. 
In this case, the depth to the coal measures under London, 
supposing they existed there, would have been from 7,000 to 
8,000 ft. or more. Speculation was hushed in presence of such 
depths, nor did it actively revive until the facts more recently 
acquired showed that the obstacles presented by such an enor- 
mous mass of strata had no real existence. 
Admitting the variation of thickness, we were hardly, how- 
ever, until lately, prepared to admit how rapid that variation 
was. Professor Hull has shown that the Lias and Oolites become 
much thinner as they range eastward from Gloucestershire. 
There is reason to believe, in fact, that the oolitic series do not 
extend far under the chalk hills of Berkshire, and it is known 
that the Inferior Oolite thins out even before reaching Oxford. 
Mr. Hull gives a section, from Gloucestershire (the neighbour- 
hood of Cheltenham) to Oxford, in which he shows that all the 
rocks below the Great Oolite thin out rapidly to the south-east ; 
