448 Dr. Mac Culloch on the Geology of 



Ketterin,* the nature of the rock alters, and it assumes an aspect 

 approaching here and there to mica slate, but it is still characterized 

 in most places by the true graywacke structure, that is to say, by 

 grains united by a cement, the cement in this case consisting of 

 micaceous schist instead of clay slate. At Ben Ledi the structure 

 still more resembles that of mica slate, insomuch that but for the 

 previous examination of the adjoining district, we should not sus- 

 pect it to consist of any thing but genuine mica slate. Beyond this, 

 and to the east of Loch Lubnaig, occurs true mica slate which 

 constitutes the whole of this district as far as Perth. I may 

 remark too, that the graywacke of which the character is so perfect 

 in the lower parts of the vale of Loch Ard, becomes micaceous as 

 we ascend the course of the stream, and that in the upper parts of 

 the valley, it is often very difficult to ascertain under which head it 

 is to be ranked. 



Those who have been engaged in similar observations on an 

 extensive tract of country, well know how difficult it is to fill up 

 all the chasms which the nature of the ground forms in the pursuit 

 of a series of rocks. Yet I have very little doubt that a true gra- 

 dation exists here, between mica slate and graywacke slate, and that 

 the usual intermediate member, primitive clay slate, does not occur 

 as a necessary part of the series, but that it is irregularly alternated 

 with the graywacke. It was with a view to these rocks, that I 

 suggested the propriety of extending the definition of graywacke, so 

 as to include those which contain mica slate, talc slate, and chlorite 

 slate, as the cement of the grains, instead of limiting it to clay slate 

 alone. In many of the rocks vv^hich occur here, the cementing in- 

 gredient appears to consist of these several substances, the rock in 



* Kcttcrin the same as Kerne used by Shakspeaje — banditti; hence Loch Kettcrin 

 and not Kathrine or Catharine as it is erroneously spelled. 



