48 Correspondence — Mr. J. Nolan. 



The Eev. 0. Fisher pointed out to me that the greater depth could 

 hardly be qualified by the word " much," and with his reasoning I 

 for the most part agree. 



I should, however, like to point out that the difference in tempera- 

 ture beneath the land and sea has by him, I think, been somewhat 

 under-estimated. He takes the mean temperature of England as 

 being 50° F., whilst that of the sea-bottom is 32^. 



If we remember that the greater number of active volcanic bands 

 lie within or near the tropics, we shall be compelled to take the land 

 temperature at something above 50° F. 



In Tokio, as recorded at the Yamato Yashiki Observatory, the 

 mean temperature at a depth of 10 ft. is about 60° F. Farther 

 south it will probably be much greater. This will make the solid 

 crust beneath the sea more nearly 2000 than 1000 ft. thicker than 

 that beneath the land, and this as a fractional part of the zone above 

 rocks at the melting temperature I regard as a considerable amount. 

 Even accepting Mr. Fisher's estimate of 900 to 1080 feet, the 

 reason I have advanced for the peculiar position of volcanos will, I 

 think, still hold good, if not for the whole of the phenomena, at 

 least for a considerable portion of it. John Milne. 



Imperiai, College of Engineering, Tokio, 

 Japan, October lOth, 1880. 



ON THE OLD EED SANDSTONE OF THE NOETH OF IRELAND. 



Sir, — In the November Number of the Geol. Mag. Mr. Kinahan 

 makes some remarks on a paper of mine bearing the above title, to 

 which I beg to make the following reply. 



I am at a loss to know how Mr. Kinahan learns from the Survey 

 Map that the Old Red Sandstone at any place " graduates " into the 

 " fossiliferous Pomeroy Eocks." No fact is more clearly shown on 

 that map than that these widely different formations are uncon- 

 formable. The conglom'erate in the townland of Aghafad I believe 

 to be of Lower Silurian age. 



I do not deny that there may be representatives of the Kiltorcan 

 beds in the North of Ireland, although I have not hitherto recognized 

 them, believing, for the reasons stated in my paper, that the " Yellow 

 Sandstones " of that district, characterized by the occurrence of 

 Modiola McAdami and other marine fossils, are far more probably 

 on the horizon of the Calciferous Sandstone of Scotland, and the 

 Carboniferous Slate and Coomhola grit of the South of Ireland. 

 As to the position of these latter groups, I beg to refer Mr. Kinahan 

 to Jukes's Manual of Geology, where he will find them placed as I 

 have done — at the base of the Carboniferous Limestone, and corre- 

 lated with the Calciferous Sandstone. 



The word " Upper " prefixed to " Old Eed Sandstone of Water- 

 ford " is simply a mistake in the abstract of my paper, which does 

 not occur in the original. J. Nolan. 



47, Great James Street, Londonderry, 

 November, 1880. 



