444 Dr. Mac Culloch on the Geology of 



of diserimmatlon will otherwise be sacrificed to the maintenance of 

 »n hypothesis, and the student whom it is a duty incumbent on our 

 Society to assist, will learn to adopt that hasty and slovenly nomen- 

 clature, which is destructive of correct description, and scarcely less 

 inimical to accurate observation. 



Having said thus much on the very ill apprehended and often 

 ill applied term graywacke, I shall be pardoned for suggesting 

 the propriety of limiting it by a certain fixed definition. Dif- 

 ferent observers have classed under it, substances the most dis- 

 cordant, looking either to their general geological hypothesis, or 

 finding it a convenient repository of rocks for which no other 

 name was at hand. Thus it has become a chaos of ill associated 

 substances. Because Cumberland and Wales are supposed countries 

 of transition, almost every rock found in those districts has been 

 occasionally called graywacke, and thus we have had breccias of all 

 possible modifications, sandstones, and clay slates, comfounded 

 with genuine graywacke under one common designation. 



The definition of Werner appears precise, and I believe I do 

 not misapprehend it, when I state that its essential part is to possess 

 clay slate as the cement of certain mechanically altered grains or 

 fragments of diff'erent rocks. These may vary materially in size, 

 and thus form the two leading varieties of fine and coarse graywacke, 

 and if they also possess a fissile structure, they will then constitute 

 fine and coarse graywacke slate. It is true that in the definition 

 of Werner, as given us by Jameson, the grains are stated to be 

 quartz, indurated clay slate, and flinty slate, but since felspar 

 and fragments of other rocks do occasionally occur in the best 

 characterized graywacke, it would probably be desirable to extend 

 this part of the character so far as to include all grains and fragments, 

 of whatever nature they may be, and to consider the cementing 



