variety, but in many places great beds of limestone full of madrepores, are contained in the slate, the limestone and slate towards the external part of the beds being interstratified. Veins of quartz, which are often of great magnitude, are of constant occurrence, being sometimes accompanied by calcareous spar and ferriferous carbonate of lime; veins of sulphate of barytes are not uncommon. Thin layers, composed of quartz, chlorite, and ferriferous carbonate of lime, are often interposed between the strata of slate, and pyrites is sometimes disseminated through the mass of the rock. Copper, in the state of sulphuret and of malachite, and veins of hematite, are frequently found, and nests of copper ore of considerable magnitude have been found in the subordinate beds of limestone.
Those who are acquainted with the geology of Devonshire and Cornwall will recognize in these characters a great similarity between the rocks which I have been describing, and those which form so large a portion of the western counties, and which have of late been designated by several mineralogists by the term grauwacke. I am fully aware of the unwarrantable extension of this name, and of the great want of precision which has been the consequence of applying it without pointing out the mineralogical structure of the compound; but I feel in common with many others the difficulty of finding a less objectionable term by which the series of rocks in question may be distinguished, when it is necessary to speak of them collectively. As the word by itself conveys no theory, and as these rocks have a closer connection with that class to which the term was originally applied than with any other, I shall call the series of rocks which I have described, a grauwacke formation, hoping that the description I have given will, in some degree, remove that want of precision which is the
