We thus arrive at the most extensive tract of all, namely, that which forms an entire belt across the island, nearly marking the separation between the Highlands and Lowlands, and reaching from sea to sea. The northern boundary of this is so nearly co incident with the southern one of the slate belt for merly described, that what marks the one will define the other. If for its breadth we commence on the cast coast, we may consider it as bounded by the Tay, and thus extending in a very irregular line, much in terrupted, and covered by trap rocks in several places till it reaches Cantyre, thus including Arran and Bute, and skirting the southern shores of the Clyde beyond Greenock, till it is also there obscured by the trap rocks of that country. In Cantyre, it similarly termi nates in a few insulated portions, marking an extent of superficies once more considerable.
In many parts of this tract quarries are wrought, and they might be opened in many more by those who are yet unaware of its immediate vicinity, and thus send to distant,quarries for what lies at their own doors. The celebrated quarries of Kingat die lie in this rock. We shall only here further remark, that it varies much in its general character, being often grey or white as well as red; and it is equally variable in hardness.
If we now examine the south side of the Frith of Forth, we find another considerable portion which bounds with the slate of the Lammermuir, and reaches towards Edinburgh, where it sinks beneath the coal strata. This portion is very familiar at Dunbar. It appears yet unsettled by geologists whether all the red sandstone of Lanarkshire belongs to the superior stra ta, or whether some may not belong to this, and thus for the present it must remain.
Equal disputes have existed respecting those por tions which appear in the south of Scotland; but they will probably be easily settled. On Mr. Bald's autho rity we have to state, that the red sandstone of Dum fries-shire is beneath the coal, and it must therefore belong to this rock. That in Eskdale is above it, and belongs consequently to the red marl of England, as must that of Roxburghshire, and probably of Ber wickshire, since coal has now been found beneath certain parts of it.
Vie thus arrive at the coal strata, which are, with little exception, the uppermost of the stratified rocks of Scotland. We have already referred to Mr. Bald's article for the minuter parts of this subject, and shall therefore merely indicate the general places and ex tent of these strata.
The coal field of Sutherland is the most northern, forming a very narrow belt along the shore at I3rora. It possesses this great singularity, that it lies almost immediately on granite, and sometimes in absolute contact. There are three beds of coal, of which one is
wrought ; and except that it contains perhaps an unusual proportion of limestone, it does not material ly differ in apparent character from other coalfields.
The next independent portion we shall notice is that of Campbelltown, occupying a very small space on the western shore, and containing one bed of very in different coal. It appears to lie immediately on mica slate, but displays no peculiarities which we can here afford to detail.
The most troublesome and worthless, yet perhaps to a geologist the most interesting, is that collection of coal strata which is dispersed through many of the western islands, and for the minuter details of which we must again refer to the work formet'ly mentioned.
This is found in scattered fragments through Sky, Rasay, the Shiant isles, Egg, Muck, Mull, and Mor ven; but it no where produces workable coal, though some trifling portions have been raised in Sky and Mull. It is by means partly of the trap rocks by which it is overwhelmed and intersected, and partly by its insular position, that its connexions become so difficult to trace, and further, that whatever coal it may contain is worthless. When the coal itself is not found, this field is traced by the limestone beneath it and in it, by its sandstones and slates, and occasionally by the coal itself. Of this latter there are numerous In dications in Sky as well as in Mull; and it also appears in Morven in the very singular situation as formerly mentioned, insulated as the summits of mountains of gneiss.
On these three latter, we must now, however, re mark, that we do not consider them as belonging to the proper coal formation immediately to be described, but to be situated in the strata above, instead of be low the magnesia]] limestone, and apparently in the lias, or some analogous member of the oolithe series. Thus, according to our views, they must be ranked with the lignite formation, in which also we place that of Yorkshire, to which they seem analogous.
The great and valuable coal field of Scotland is that which is well known for its workings. The northern limit is also the boundary of the red sandstone already described, and from this it crosses to the similar boundary south of the Forth, though in many places covered and obscured by ranges and hills of trap. From Edinburgh, it proceeds partly to the south of the Pentland hills, where it terminates, and to the north of these westward to Glasgow. Thence diverg ing into Clydesdale, it is interrupted by the trap hills of Renfrew and Ayr, reappearing on the western shore, till it terminates in the red sandstone and the slate of that district.