which happens to suit any particular hypothesis which we may wish to support. The accurate description of mineralogical species, must be the base of all geological reasonings, but if we intermix characters derived from geological circumstances with true mineralogical characters, we set out upon a petitio principii, and end by reasoning in a circle. In the case of limestone, the chemical properties and well known popular characters of which have given it a very decided and constant name, we act rightly. The name of the species is retained, be its geological situation what it may, and its geological accidents are distinguished by the addition of that name which suits its position, whether primitive, transition, or flœtz. But on the other hand, a mineral equally common, whose characters too are sufficiently familiar, is distinguished by two names, although even more identical in structure than the different tribes of limestone, and it is called clay slate, or graywacke slate, for no other reason than that in the one case it is associated with the rocks called primitive, and in the other with those which go by the name of transition. It is obviously necessary that the same practice should be adopted in this case as in the former, and that, adhering to the name of the species, we should distinguish its geological position, if required, by the superaddition of the corresponding term. We shall thus have primitive clay slate, and transition clay slate, if we find it necessary to retain these geological distinctions.
It is almost superfluous to quote instances of the existence of genuine clay slate occurring together with well characterized graywacke slate, as they may be seen in all the slate counties of England, and among others remarkably in Cornwall. I trust that the geologists who have attended to these rocks, will see, with me, the necessity of adopting this distinction; a distinction, without which all accuracy