To omit noticing a few patches and minuter strata of this rock so scattered and so trilling as to be im practicable in detail, we must now pass suddenly from the north to that immense tract of the same rock which may almost be said to form the south of Scot land. Here again we are lost in attempts to define its boundaries; and, as formerly, must trust partly to the account we shall have to give of the sandstone, which forms the chief exception.
But of this rock, we must first remark, geologically, that its general characters are very different from that of the clay slate in the northern division, as we partly indicated in treating of the physical geography of that district. There is here no such regular sequence from granite upwards, nor no superposition of this clay slate to mica slate and gneiss, as so often occurs there. In fact, its real geological connexions and po sitions are as yet very obscure; but in this it partakes with Wales, and the Isle of Man, Cornwall, and Cum berland, which, in all important particulars, it resem bles. It is probable that it must be esteemed, like those, to follow immediately on granite, with few or scanty portions of other rocks interposed; and that there is thus but a brief and imperfect series between the lowest of the primary and the lowest of the secon dary rocks.
In its mineral character also, it somewhat differs, as it does in the accuracy of its stratification and of its lamination, or fissile tendency. Hence it is difficult to discover and pursue the stratification; and as the la minar tendency is equally rare, or irregular, or obscure, it seldom is capable of affording roofing slate. Yet we must observe that it has not been sufficiently examined for this purpose, and is indeed, in most places, scarcely suspected of being the very substance from which slate is generally procured. With respect to its quality or texture, the coarser kinds, formed of fragments more or less minute, prevail very much; yet these are susceptible of being wrought for slate, as they actually arc in Cumberland; while even among the coarsest rocks, beds of a perfectly fine quality are often interposed.
To define this great tract of clay slate to the north ward, it is necessary to commence with St. Abb's head, and to pursue the course of the Lammermuir• hills, still continuing to skirt the mountains till we reach the sea coast near Ayr. Hence westward, east
ward, and southward, Scotland alone is its boundary, if we except the intrusions of the northern coalfield, the red sand stone of both kinds, beneath the coal in one case and above it in the other, which enters near Berwick from England, as well as in Roxburghshire along the course of the Esk, at Jedburgh, and into Dumfries-shire or Galloway; tracts which we find our selves unable to define in words by any species of to pographical reference.
The granite already mentioned is also, of course, excepted. The general elevations and characters of this country were formerly given, and we may thus terminate the history of this important rock in Scot land.
Among the primary rocks, besides limestone, it re mains to notice one or two which occur in very small quantities, and which are rather objects of geological curiosity than of general or economical interest.
The first of these is serpentine, sparingly dispersed everywhere, though found in many places. One of the most remarkable, and the most extensive tracts of this rock, occurs in the Shetland islands, where, how ever, it is nearly limited to the two northernmost, Unst and Fetlar. It here forms a body of considera ble extent, stratified among the other primary rocks, and accompanied by diallage rock. It forms lofty and bold broken cliffs, and rises into hills considerable for these islands. It is chiefly remarkable here for the minerals which it contains, of which chromate of iron is the most conspicuous; a substance which might have been of great value in the arts of dyeing and colour-making, had sot the market been more easily supplied from America. It also produces two rare minerals, the hydrate of magnesia and oxide of chrome, of which the latter has hitherto been found nowhere else. As far as has yet been observed, it contains no ornamental portions, being generally of a uniform dull gree14; but it is very possible that these may ex ist in it.
As the diallage rock is known only here in all Scot land, we shall now mention it, that we may not have to recur to Shetland again immediately. This rare substance is intimately associated with the serpentine, forming the whole of the small island Balta, and parts of Unst and Wya, and being found also in very small portions on the mainland of Shetland.