292 FOSSIL FLORA OF THE 



the Lower Silurian formation. The most remarkable coal in the 

 Secondary series is a rich bituminous bed, 36 feet thick, in Eastern 

 Virginia, which Sir Charles Lyell has shown to be not more ancient 

 than the Lias formation. But, with these and a few other exceptions, 

 productive or valuable seams of coal are found only in that formation 

 which lies between the Old Red Sandstone and the New Red Sand- 

 stone, and which has been called Carboniferous, because a large 

 amount of carbon is locked up in the coal and limestone beds. Now 

 this conclusion is of practical value, enabling the Geologist to point 

 out the class of rocks which will yield coal, and also to prevent fruit- 

 less and expensive boring and sinking in formations, which he knows 

 are unproductive of coal seams. 



Associated with coal are groups of strata which are repeated, with 

 some slight variation, as often as a coal-bed occurs. These strata 

 are shales, or mud which had been deposited by water; sandstones, or 

 grains of silica or sand, such as are seen on the sea-coast, bound 

 together by a cement, which may either be silica itself, lime, or oxide 

 of iron ; not unfrequently ironstone nodules are sticking in the shale 

 like plums in a pudding. In Northumberland, sandstone occasion- 

 ally forms the floor of a coal for a short distance ; but in other parts 

 of Great Britain, in America and in Nova Scotia, a peculiar bed of 

 " under-clay," which, in most instances, is a good fire-clay, lies 

 beneath every coal-seam, and is penetrated in all directions by a 

 singular fossil plant. 



The Carboniferous formation may be divided into two sections — 

 the Coal-measures distinctively so called; and the Mountain Limestone, 

 which in some districts, as in Derbyshire, yields no coal, but which 

 in Northumberland, Berwickshire, and Russia contains coal-seams of 

 some value. We shall take these sections as they occur in Northum- 

 berland and Berwickshire, to show their distinctive difference. If a 

 line be drawn from the mouth of the Aln to a point a little east of 

 Hexham, the district south-east of that line would be the coal- 

 measures, which have, at their base, a sandstone of considerable 

 thickness called the "millstone grit;" but the remainder of the 

 county would be, generally speaking, the mountain limestone, which 

 passes underneath the coal-measures, and which had been deposited 

 before them. The porphyry of the Cheviots has been protruded 

 through the sedimentary rocks in the western part of the county ; a 

 tongue of Silurian beds, highly inclined, abuts against this porphyry 

 near the head of the Coquet ; and on the eastern flank, there yet 

 remains a patch of Old Red conglomerate. Several dikes of basalt 

 cut through the mountain limestone beds, and the same volcanic rock 

 is interstratified with them, and presents, in several places, bold cUff- 

 faces towards the west. The Mountain Limestone is continued into 

 Berwickshire in a narrow strip along the coast to near Burnmouth ; 

 it also covers a considerable portion of that county north-west of 

 the Tweed, forming something like an arc of a circle, of which 

 Dunse is the summit, and Berwick and Kelso the extremities of the 

 chord. It has for its base chiefly old red sandstone, which occupies 

 the middle portion of the county, and rests unconformahly on 



