The rocks which are the subject of this paper are not all of them strictly claystones. That term is restricted by mineralogists to the softer varieties of felspathic rocks; but as it is impossible to draw any line of demarcation between these softer varieties, and many other closely related rocks, and as such a line of demarcation, if drawn, would be of no avail towards our present object, but would rather be a stumbling block and a hindrance, I have taken the liberty of referring to all that ever-varying group of the Arran rocks, of which the base is a felspathic paste, under the familiar Saxon term which stands at the head of this paper. Not that there is any connection existing between these rocks and the substance familiarly known as clay; for like many other terms in common use among geologists, the relationship between the name given, and the object referred to, is not in this case the obvious one; but because the simplest kind of rock belonging to this whole series goes by that name. Mineralogically, claystone is a compound of quartz and felspar, and is chemically composed of silex, alumina, and potash—silex, of course, greatly predominating over the other two substances, and alumina prevailing over potash in the proportion of about 2 to 1. The quartz in these claystones stands in various relations to the felspar; sometimes they are both thoroughly mingled together into a felspathic paste, and then we have a rock which is called claystone This 250-word extract was created in the absence of an abstract