Correspondence — Messrs. Pengelly, Penning, Ward. 383 



TRUE AND APPARENT DIP.— FLOATING ICE. 

 Sir, — "The accurate rule" given by Mr. E. Hill, in tlie July 

 number, Geol. Mag. p. 334, is, I believe, identical with that given 

 by the late Professor Phillips in his Guide to Geology, 5th ed. (1864), 

 p. 298. 



Whilst writing, I may as well call attention to what appears to be 

 a misprint in Mr. J. Milne's paper, in the same Number. The 

 equation " H = /i = 1-080," p. 307, line 6 from bottom, should, T 

 think, be H =: ^ X 1-080. W. Pengelly. 



Lamobna, Torquay, 5 July, 1876. 



TRUE AND APPARENT DIP. 



Sir, — ^I am obliged to your correspondent, the Eev. H. G. Day, 

 for his remarks on my rules for finding the direction of true dip. 

 I was aware that there would be a slight error arising from the 

 difference between the circular measure of an angle and the tangent 

 of that angle, but in practice the former is much more readily adopted, 

 and the error is so small that generally it may be ignored. 



My friend, the Eev. 0. Fisher, writes me, " Your rule A is correct 

 for small dips for all practical purposes. It is not necessary that 

 the angle between the faces of the quarry should be obtuse, the rule 

 is equally true whatever be the angle." 



The modification Mr. Day proposes for rule B is an improvement : 

 but it would be better still, as Mr. Fisher had in the meantime 

 suggested, to measure off a line parallel to one face of the quarry 

 and apply rule A behind it, — adding, "you may step it on the ground 

 at once, without making a diagram." W. H. Penning. 



H.M. Geological Survey, 

 loth June, 1876. 



ABSENCE OF LLANDOVERY ROCKS IN THE LAKE DISTRICT. 



Sir, — I venture to think that Mr. Hicks's somewhat authoritative 

 treatment of Mr. Aveline's opinion — and Mr. Aveline's opinions are 

 never hastily formed — with regard to the absence of the Llandovery 

 rocks in the Lake District, appears a little out of place, when it is 

 remembered that Mr. Aveline has spent nearly a lifetime among 

 the rocks of Wales and their equivalents in Cumberland, and that 

 Mr. Hicks knows nothing of Cumberland, though he is justly 

 regarded as an authority upon many of the rocks of South Wales, 

 among which he has worked so ably and successfully. 



I cannot but think, also, that the fossil evidence, upon which so 

 much stress has been laid, must not be received unhesitatingly as 

 absolute. The presence or absence of certain Graptolites in these old 

 rocks is but imperfect evidence at the best, so little do we yet 

 know of the life-history of this group, and so much have recent 

 deep-sea discoveries shaken one's faith in the absolutism of palte- 

 ontological evidence. 



Had Mr. Hicks worked among the Cumberland rocks the number 

 of years that Mr. Aveline and I have done, he would not perhaps 

 have found things so " tolerably clear " on comjoaring the districts of 

 Cumberland and Wales. J. Clifton Ward. 



EsKDALE, June lOih, 1876. 



