25 
formation, and between these and the fourth lies the great store of 
Edinburgh coal. In this interval of 1200 feet occur 26 workable seams 
with a total thickness of 80 feet. Into this subterranean treasure-house 
the pits that stud the Dalkeith coalfield are sunk. These beds underlie 
the whole district, cropping out at a high angle on the flank of the 
Pentlands as the Edge coals of Mid Lothian, and again at a much smaller 
one about 5 miles to the eastward where they are called for distinction 
the Flat coals. 
Before leaving this part of the subject it may be interesting to add that 
the district was not disturbed by the subterranean forces for many ages 
after the close of the carboniferous era. They again broke forth for a 
time, probably about the date of the beginning of the tertiary deposits, and 
the top of Arthur's seat was the crater from which issued new streams of 
lava and showers of ash. Its highest point consists of a plug of basalt 
filling the pipe and remaining as a proof of the eruption and not less of 
the enormous erosion that has since taken place. 
Very different is the succession of the beds in the same formation on the 
western side of the chain though only a few miles distant. It consists, in 
the lowest part, of shale, sandstone, and ironstone, with a few limestone 
bands corresponding to those in Mid Lothian and several seams of coal of 
good quality. Above these lies the product of one or more of the ancient 
eruptions, in the form of a vast sheet of greenstone 150 feet thick through 
which the pits must be sunk before the coal is won. Above this once lay 
(for it has now been worked out) the "Wester Main coal, one of the most 
profitable seams in the field, 12 feet thick, and this in turn was covered by 
an upper greenstone often 250 feet in thickness. 
In one pit a deep sinking was made to the Wester Main coal through 
150 feet of this greenstone, when a series of shales and sandstones was 
reached. Greenstone followed and was pierced to a depth of 140 feet 
when the auger struck a rich seam of coal 9 feet thick. The pit was 
deepened to this point and the coal worked. Its quality was good, though 
underlain and covered with greenstone. But it was soon found that the 
seam formed but a local lenticular patch filling a hollow in the trap, and 
though mines were driven into the rock in every direction no trace of coal 
could be found. The boring was therefore resumed, but after passing 
through an apparently interminable thickness of greenstone at great cost, 
the pit was abandoned.* 
* Memoir of Survey. 
