408 P ro f' Hetisloib's Reply to Dr. Berger. [June, 



in the formations of clayslate, and the infinite varieties of grey- 

 wacke. Unless, therefore, a distinction can be established 

 from geological position, there can be no reason for separating 

 them. It is possible such a distinction may exist in the Isle of 

 Man, and I am even disposed to think it does, though the limits 

 assigned by Dr. Berger appeared to me so very unsatisfactory 

 that I preferred designating the whole on the map by an uni- 

 form colour. The propriety of this method has been confirmed 

 by the learned and experienced geologist Dr. Macculloch, who 

 has adopted precisely the same plan, in his account of this 

 island, in the second volume of his History of the Western Isles 

 of Scotland, published after my paper was transmitted to the 

 Society, but before it had been printed. 



Should Dr. Berger wish for further proof that he ought not to 

 consider himself infallible on points of geology, I beg leave to 

 refer him to Prof. Sedgwick's paper on the Geology of Corn- 

 wall, published in the Transactions of the Cambridge Philoso- 

 phical Society.* 



5. Indistinctness with regard to my " quartzose districts." 



To this I am desirous of pleading guilty. I now feel but little 

 doubt that these districts are merely subordinate to the clay 

 slates. Want of experience, at the time I examined them, led 

 me to suppose (from the marked difference of external character) 

 that they might belong to a separate formation. 



C. The correction of the line of boundary assigned by Dr. B. 

 to the mountain limestone, or I should rather say boundaries, for 



* I may refer more particularly to the Professor's remarks in p. 195, but shall con- 

 tent myself with a few extracts from pp. 13'J, 140, and 141, as they happen to be 

 directly applicable to the case in point. 



" The rocks hitherto described in this section are considered by Dr. Berger as belong- 

 ing to the greywacke formation ; which, according to that author, extends, almost 

 without interruption, on both sides of the central chain. To prevent any ambiguity, he 

 first defines greywacke, &c. and he then divides the formation into common greywacke 

 and greywacke slate. He afterwards adds, that in Cornwall, common greywacke is 

 always found higher than greywacke slate, &c. — that it rests immediately upon the 

 granite — that it is much less rich in ores than greywacke slate, &c. He then states — 

 that the greywacke slate becomes more perfectly schistose as it is farther removed from 

 the granite — that its base is fine, smooth, and nearly homogeneous — that it sometimes 

 possesses the lustre of satin — that it is to this second variety exclusively that the Cornish 

 miners give the name of killas, &c. 



" We cannot help considering the whole of this account as inadequate, and in a great 

 measure inapplicable. No one term is sufficient to characterize the various beds of 

 this formation, &c. If we assume with Dr. Berger, that common greywacke rests 

 immediately on the granite ; still it is not true that it is always found higher than the 

 gTeywacke slate, &c. Again, if the common greywacke be much less rich in ores than 

 the greywacke slate, then the part of the killas which rests immediately on the granite 

 must be much less metalliferous than some other parts of the formation, and more 

 remote from the fundamental rock. As a general observation, we believe this to be 

 untrue, &c. But this is not all; we will venture to assert that the rock in immediate 

 contact with the granite (at least in every instance in which we have ourselves examined 

 it) bears no resemblance whatever to common greywacke. With equal impropriety we 

 conceive the term greywacke slate applied to all the finer schistose beds, of silvery 

 lustre, which abound so much in this formation : because, &c. Lastly, it is not true, 

 that the Cornish miners apply the term killas exclusively to these finer schistose beds, 

 &c. &c." 



