4 GEOLOGICAL SURVEY OF ALABAMA. 
ern (Central) to Thompson’s Gap, or to Carr’s Gap on the 
Georgia Pacific, both on Big Oak Mountain and near Leeds, 
(these last mentioned gaps being in the Coosa Millstone 
grit,) you will find them all similar. You will find the same 
rock at the top of Monte Sano, Huntsville, at the top of 
Lookout Mountain, Chattanooga, and all along the base of 
the Coal Measures of Walden’s Ridge and Sequatchee Val- 
ley, Tennessee ; you will also find the same rock at the 
base of all our Alabama Coal Measures wherever they are 
the country (or surface) rock. 
The underlying Sub-Carboniferous limestone is not very 
thick near Brock’s Gap, becomes thicker going Northwards, 
as is evident at Blount Springs, where Col. Jackson opened 
his quarry, and to agreater degree still at Huntsville, where 
it is over 700 feet in thickness. It also shows a great thick- 
ness at Chattanooga. 
The Coal Measures of the Cahaba Coal field, like those 
of the Indian Territory, have only one thin ledge of lime- 
-stone a few feet in thickness, in the whole of the measures ; 
in both places it is arenaceous and near the middle of the 
measures.* Richard P. Rothwell, Editor of the Engineer- 
ing and Mining Journal, New York, was the first to discover 
this ledge some twenty years ago. The almost entire ab- 
‘sence of limestone in our Coal Measures is one of the main 
points of difference between them and those of the North- 
ern and Western States. Another great peculiarity in our 
Alabama Coal Measures, in which they differ from anything 
seen by the writer in the United States, England, Scotland, 
Wales and the Continent of Europe, is that the great con- 
glomerate of our Coal Measures is at the top of the series. 
The five hundred feet of measures above the Montevallo 
seam are mostly conglomerates or pebbly sandstones (for 
description of which see chapter on Montevallo basin). 
I have no knowledge of any similar case except the Coal 
Measures near Sydney, Australia, where the top rock of 
their measures is an immense conglomerate, still larger 
than ours. 
The resemblances between our Coal Measures and those 
* A ledge of limestone similar to that described by Mr. Squire is 
found also in the Warrior Coal Field. EA. 8. 
