Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/296

 bed of two or three feet in thickness. In its general aspect it so nearly resembles a granular quartz as to be at first sight difficultly distinguished from it. As it produces no effervescence with acids, it probably contains no great mixture of carbonate of lime, and may therefore be considered as a granular tremolite. It may be distinguished from quartz by its superior lustre, translucency, and specific gravity, as well as by its inferior hardness. The sp. gr. of the specimens which I tried was 2,840.

A fifth variety is found in the same place, but without so very marked a character as that of the one preceding. It is foliated, with a slight tendency to a fibrous structure, and forms large concretions in the marble. Its aspect is vitreous, and it is of a greenish grey colour.

Another modification of this beautiful mineral which occurs here can scarcely be considered of sufficient importance to constitute a mineralogical variety. It splits into thin laminæ, of which the flat surface has the splendour and almost the uniform smoothness of white satin, but on examination it is seen to consist of very minute fibres of tremolite interfoliated with very white and compact carbonate of lime. Some specimens are again found which seem to consist of a mixture of tremolite and schiefer spar, a substance which is also found although but rarely, among the calcareous beds.

The various forms indeed under which the several mixtures of tremolite with the surrounding rocks occur are such as to elude description, and not a little to puzzle a mineralogist. I shall only enumerate two more of the most remarkable. One of these is a compact schistose rock in which the union of the tremolite with the limestone is so intimate that it can only he distinguished in the first instance by its hardness and specific gravity, as the eye cannot detect the mixture. In the other the tremolite is mixed with a kind