150 PEOF. EDWAED HULL, LL.D., F.E.S._, F.G.S., ON 



few that are more remarkable than this. The origin of them is 

 always mysterious, but this explanation given by Professor Hull 

 seems to be very reasonable and feasible. In general, though, I 

 have observed, and it has been my business to observe, in Australia 

 and the East a good many mining operations, and in those 

 frequently tunnels are driven and shafts are sunk in the decomposed 

 granite. Invariably it is found to be the case that decomposed 

 granite always contains some harder portions which are met with in 

 a way which I am quite unable to explain. 



I should like, before I sit down, to ask Professor Hull about the 

 Logan Rock mentioned in his paper. My acquaintance with 

 England is somewhat old, and I have forgotten some of the things I 

 have read ; but my impression is that the Logan Rock was, for a 

 long time, understood to be an artificial monument — that it was a 

 rocking stone, in fact, balanced finely upon a point, and that it was 

 dislodged from that point by some mischievous persons a good many 

 years ago. I would like to ask Professor Hull if the Logan Rock 

 to which he refers is the one of which I have a vague recollection. 



Mr. David Howard. — I think that everyone who knows Devon 

 and Cornwall will appreciate the extreme interest of this paper, not 

 merely from the question of the particular Cheesewring it deals with, 

 but the light that it throws on the whole geology of those counties, 

 or, to use that dreadful expression, their "physical geographj^" 

 Nothing strikes me so much as the dulness or flatness of the upper 

 level of Cornwall, and the wonderful beauty of the valleys cut out 

 in the lower portion at a much later period. Denudation has been 

 at work in two such totally different ways, a contrast which I think 

 of great importance, because very often, without a special knowledge 

 of geology (and a little knowledge of this branch of science, like others, 

 is a dangerous thing), it leads one to suppose that we must look 

 only for denudation which is at work at the present day. In Devon 

 and Cornwall, and especially in Cornwall, there is an old sea-bottom, 

 so to speak, and you see the rocks standing up by themselves, 

 leaving a comparatively flat floor, and in the lower portion you see 

 the effects of the rapid running water flowing down from a higher 

 level and eroding the valleys on its way to the sea. 



As to why granites are sometimes harder and sometimes softer, 

 we are met with the great problem how granite came to be granite 

 at all. The construction of its crystals is due not only to cooling, 



