Although a great many of the rocks of Scotland belong to the Silurian system, most of them belong to the lower division, those belonging to the upper divisions being best developed in Lanarkshire and in the sections occurring in the Pentland Hills. When Edinburgh was the centre of the controversy between the Huttonians and the Wernerians, the rocks of the Pentlands were doubtless often explored and discussed. These for the most part were the igneous rocks of which they are mostly composed. But it is in Mr Maclaren’s ‶Geology of Fife and the Lothians,″ under the name of ‶Vertical Greywacke,″ where we first get anything like a detailed account of them. We are also indebted to Mr Maclaren for the first notice of fossils from them, he having discovered an Orthoceras in the beds. From this, and another specimen found by Mr Geikie, Sir R. Murchison, in the last edition of his ‶Siluria,″ speaks of them as Wenlock; but, from the materials at his command, Murchison’s determination of the age of the beds could be little more than a guess. From the publication of Mr Maclaren’s book, the Silurian rocks of the Pentlands seem to have excited little interest until the publication of the Memoir of the Geological Survey, which gives us a short but very good description of the beds, and in the appendix we get a list of the then known fossils, accompanied by a plate. Mr Salter’s list contained several new species, which he could not describe,