S44 Mr. Ho'HUER on I he Geology of the 



variety, but in many places great beds of limestone full of madre- 

 pores, are contained in the slate, the limestone and slate to- 

 wards the external part of the beds being interstratified. Veins of 

 quartz, which are often of great magnitude, are of constant oc- 

 currence, 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 ma- 

 lachite, 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 be- 

 tween 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 grau^ 

 wacke. I am fully aware of the unwarrantable extension of this 

 name, and of the great want of precision which has been the con- 

 sequence 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 forjnation^ hoping that the description I have given 

 will, in some degree, remove that want of precision which is th« 



