The second class of contemporaneous trap-rocks are the tufas or volcanic ashes. They differ entirely in their aspect and origin from any of the rocks already described. Greenstones, basalts, felstones, and such like, were all melted rocks, thrust up from below as we see lava thrown out by a modern volcano, being styled contemporaneous when poured out along the sea-bottom or the land, and intrusive when they never reached the surface but cut through the strata below. The tufas, however, point to a totally different origin. They are of various shades of colour, according to their chemical composition. In East Lothian they assume a deep red hue; among the Pentland Hills they are often flesh-coloured, while in Linlithgowshire they range from a dull-brown to a pale leek-green, green being the prevailing tint. They always show a dull uncrystalline surface, irregularly roughened by included fragments of various rocks, such as trap, sandstone, shale, and many others. These fragments or _lapilli_ vary in size from less than a pin-head up to large bombs of several hundredweight, and from being generally abundant give to the tufas one of their best-marked characteristics. The smaller pieces are usually more or less angular, and throughout the carboniferous series of Linlithgowshire consist chiefly of a pale felspathic matter, lighter in shade and commonly harder in texture than the matrix or paste in which they lie. In some localities, where the included pieces are larger, they have a rounded form, and often show a honey-combed vesicular surface, like balls of hardened slag. Fragments of sandstone have not unfrequently a semi-fused appearance, and plates of shale sometimes look like the broken debris from a tile-work, although in many instances these fragments may be found showing no trace whatever of alteration, being undistinguishable from the neighbouring sandstones and shales from which they probably came. I have seen in some of the coarser tufas, or rather volcanic conglomerates, enormous masses of basalt and greenstone buried deep in the surrounding green or red felspathic paste, and showing on their more prominent edges the usual vesicular cavities. In such conglomerates there is usually no division into beds; the whole mass, indeed, forms a bed between lower and higher strata, but internally it shows for the most part no trace of stratification. In these confused assemblages one may occasionally light upon detached crystals of augite or other mineral scattered irregularly through the tufa. Their angles will be found often blunted, and the crystals themselves broken, appearances which have likewise been noticed among the ash of modern volcanoes. When the tufas are finer grained they usually exhibit a well-marked stratification, and can often be split up into laminæ like an ordinary fissile sandstone. Organic remains not unfrequently abound in such laminated beds, and vary in their character