382 Correspondence — Mr. James Croll. 



ON THE EELATION OF THE PORPHYRY SERIES TO THE SKIDDAW 

 SLATES IN THE LAKE DISTRICT. 



Sir, — In the Geological Magazine for February last, there is a 

 paper by Mr. J. E. Dakyns on a supposed unconformity of the Por- 

 phyry series on the Skiddaw Slates. When I read the paper I 

 doubted very much the correctness of Mr. Dakyn's observations, and 

 since then I have, along with my colleague (Mr. J. C. Ward, who is 

 now surveying that part for the Government Geological Survey), ex- 

 amined the line of junction between the Porphyry series and Skid- 

 daw Slates from Derwentwater to Warnscales Bottom, Buttermere, 

 and we found that throughout the entire distance the two formations 

 are brought against each other by faults of considerable magnitude. 

 I cannot enter into details, which must be reserved for the Geological 

 Survey Memoirs of that country. It is possible there may be an 

 unconformity between the two series of beds, but the evidence of it 

 must be sought for elsewhere than in the district described by Mr. 

 Dakyns. 



In reference to an additional Note by Mr. Dakyns on the same 

 subject in the March number, I beg to say there is " a fault with an 

 enormous throw" along Derwentwater. — I am. Sir, your obedient 

 servant, W. Talbot Aveline. 



Geological Survey of England and Wales, 

 Kendal, June 2Sth, 1869. 



ON MR. MURPHY'S THEORY OF THE CAUSE OF THE GLACIAL 



CLIMATE. 



Sir, — In the Geol. Mag., July 1, p. 331, you report at the meeting 

 of the Geological Society, June 9th, a paper by J. J. Murphy, Esq., 

 F.G.S., " On the Nature and Cause of the Glacial Climate." It is stated 

 that he agrees with me as to the Cause of Glacial Climate except in 

 one instance. " He maintained in opposition to Mr. Croll that the 

 glaciated hemisphere must be that in which the summer occurs in 

 aphelion during the greatest eccentricity of the earth's orbit. He 

 shewed that a cool summer had more to do with the prevalence of 

 glacial conditions than a cold winter, and referred to several phe- 

 nomena furnishing arguments in favour of his opinion." 



I fear that Mr. Murphy must be resting his theory on the mis- 

 taken idea that a summer in aphelion ought to melt less snow and 

 ice than one in perihelion. It is quite true that the longer summer 

 in aphelion — other things being equal — is colder than the shorter 

 one in perihelion, but the quantity of heat received from the sun is 

 the same in both cases. Consequently the quantity of snow and ice 

 melted ought also to be the same ; for the amount melted is in 

 proportion to the quantity of energy in the form of heat received. 



It is true that with us at present less snow and ice are melted 

 during a cold summer than during a warm one. But this is not a 

 case in point, for during a cold summer we have less heat than 

 during a warm summer, the length of both being the same. The 

 coldness of the summers in this case is owing chiefly to a portion of 

 the heat which we ought to receive from the sun being cut off by 

 some obstructing cause. 



