One of the most remarkable and difficult phenomena presented by these limestones is their great persistency. I have traced them over a large part of Mid-Lothian, from the highly inclined beds at Joppa to the contorted and faulted strata near Carlops. I have found them, too, in many parts of West-Lothian and Stirlingshire, from the sea at Borrowstounness southwards into Lanarkshire. They likewise occur in Fife, and seem to sweep away through Lanark and Ayrshire. The area in which I have found them cannot be much under 700 square miles, yet they are probably spread over a much greater extent of country. Throughout this region they appear to continue on the whole at pretty much the same vertical distance from each other, and average three or four feet thick each. They vary in number, three being found in parts of Mid-Lothian, in other parts only two. Throughout West-Lothian there seem to be but two seams in the middle or moor-rock series, and the same two seams are found passing over into Perth near Culross. There are differences, too, in the structure and composition of the seams, one running sometimes as a single bed of dull blue limestone, and then gradually splitting up into three layers of a greyer and more earthy texture, with soft shale between them. But making all these abatements, the observer cannot fail to be struck with the general regularity and continuity of these limestones. And the fact becomes all the more remarkable when we consider the great irregularity, and continual intercalations, and repetitions of the strata, both above and below. Marine beds are usually persistent over large areas, especially where extensively developed. As they decrease in thickness, their continuity for the most part lessens, so that the rule is, on the whole, a safe one, the thinner any particular stratum, the less likely are we to trace it to a considerable distance. Yet, not only are these Mid-Lothian limestones thin, but they occur in regular sequence among a set of continually alternating and very irregular beds, and extend over several hundred square miles of country. And this, too, not in a single seam, but in two, three, or even more, so that the difficulty of accounting for such intercalations is proportionately increased.