26 
Little metamorphism has taken place in the coal, even where it has been 
overflowed by the trap. It is sometimes worked with the greenstone for a 
roof, and the alteration becomes almost imperceptible at the distance of a 
few inches from the junction. It need hardly be added that this is not the 
case when the trap has intruded itself through the sedimentary rocks. In 
such cases the coal is burnt and charred at the surface of junction and for 
some distance from it. The trap, too, finding less resistance in that 
direction, has often forced itself along the line of a coal seam, and so im- 
pregnated the mineral as to render it over large areas utterly useless for 
fuel. Nor has the intruding rock escaped reaction, being often itself so 
altered in appearance as to be nearly unrecognizable. 
Following these enormous sheets of trap is a succession of beds of 
sandstone, shale, and coal, until the thin upper limestone, which occurred 
at the top of the Edinburgh field, is also met with here. No beds corres- 
ponding to our upper shales exist, but the millstone grit immediately 
succeeds the upper limestone and averages about 400 feet in thickness, 
interlaminated in some places with shale and fire-clay. As in this 
neighbourhood, it contains few or no peculiar fossils. The upper coals with 
their sandstones complete the series and correspond in position to those of 
our own field. 
In conclusion, the marked difference between the aspects of the system 
exhibited in the two localities seems to imply a difference between their 
conditions of deposition. From the top of the Old Red Sandstone to the top 
of the millstone grit there is, in our Avon section, not a trace of igneous 
action, want of conformity, or of fresh- water deposits, and except one fault, 
having a throw of about 130 fathoms, no observable dislocation. The 
whole 4000 feet is a continuous marine formation embedding contempo- 
raneous organisms. On the other hand the same system in Linlithgow 
and the Lothians exhibits 7400 feet of strata betraying rather fresh- water 
or fluvio-marine conditions of deposit. The fossil remains from the 
calciferous sandstones of Arthur's seat, the sandstones of Granton and 
Craiglcith, and the shale beds of the middle limestone, are almost all those 
of land-plants, or of sauroid fish such as may have infested the brackish 
water of estuaries.* Only in the few thin limestone bands do true 
Mountain Limestone fossils occur — shewing that the species inhabited the 
neighbourhood, and serving to render their absence from the shales and 
* The Lepidosteus and Polypterus, the only living Sauroid fish, are 
both inhabitants of fresh water; the former of the rivers of N. America, and 
the latter of the Nile and Senegal. — BxcklamVs Bridgwater Treatise, p. 274. 
