10S On Transition Rocks. [Aug. 



Mr. Allan's own words, " the shades of distinction chronicled 

 by the mineralogist cannot be expected to attract the attention of 

 the miner." Mr. Allan would surely not contend that grey- 

 wacke and green-stone are the same because they are often con- 

 founded by the vulgar under the name of whin-stone. That a 

 writer, who is at such pains to press (and justly) on geologists the 

 " necessity of minute attention " and " accurate examination," 

 as " the only solid basis of philosophic inquiry," should thus, 

 when it seems to suit his own argument, feci so much disposed 

 to pay attention to the mere appellations of the vulgar, is surely 

 rather extraordinary. 



Quid non mortalia pectora cogis, 



Theori.-e sacra fames ! 



That the rock which lies immediately on the granite of the 

 Lauran is of a very different composition from the grey-wacke in 

 its neighbourhood, is, I think, evident, not only from its quali- 

 ties as tbey appear to the eye in hard specimens; and from its 

 superior hardness and toughness under the hammer, as Mr. A. 

 expresses it; but very much from the manner in which it under- 

 goes decomposition when exposed to the air. Any person accus- 

 tomed to view these two rocks will at once distinguish in the 

 field a piece of the tine-grained gneiss from the grey-wacke rock 

 with which it is accompanied. The grey-wacke disintegrates 

 sparingly, and always nearly uniformly over the whole surface 

 exposed ; whereas the fine-grained gneiss disintegrates very 

 much, and always in an irregular manner, showing deep pits and 

 eminences, or ridges and furrowings on the surface. This, there 

 can be little doubt, arises from the felspar contained in its com- 

 position, which we know undergoes decomposition very strikingly 

 on many occasions. 



I consider myself as now warranted, from what I have said, 

 to draw the following conclusions directly the reverse of those 

 deduced by Mr. Allan : — 



" The killas of Cornwall," that is, the rock which lies imme- 

 diately over the granite of that country, " belongs not to the 

 transition series of Werner." 



" The granite of Cornwall is not possessed of every character 

 by which tbe oldest varieties are distinguished." 



" That granite, the nucleus round which Werner conceives 

 all other rocks were deposited, is not proved to be in some cases 

 actually of a later date than the transition series." 



" Hence the distinction of transition rocks is not proved to be 

 grounded on false conclusions." 



" And, finally, Werner" need not " make very material 

 alterations in his present system " in order " to accommodate it 

 to the phenomena so commonly presented in nature." 



Had Mr. A.'s paper been written in a less assuming tone, I 



