Correspondence — Mr. J. A. Birds. — Frof. J. Milne. 47 

 coI^I^:ESI^03^^DE^s^o:E. 



FOEEIGN PEBBLES ON OUR SOUTH COAST. 



Sir, — I am not aware if the significance of the foreign stones 

 — I do not mean, of course, the granite blocks at Pagham, but 

 smaller pebbles of granite, porphyry, etc., occasionally occurring in 

 the shingle of our southern coasts — has ever been remarked upon ; 

 but it has occurred to me, as no doubt to a hundred other geologists, 

 both professional and amateur, that these are almost the only actual 

 evidence that can be expected, in the absence of Drift and ice- 

 markings, in favour of Dr. Croll's suggestion as to the passage of a 

 great ice-sheet over the South-Eastern corner of England during the 

 height of the Glacial Epoch. I have frequently observed such stones 

 both here and at Brighton, and only within the last few days, I have 

 picked up a variety of granites, syenites, quartz-pebbles, and por- 

 phyries (two or three dozen in all), some red sandstones, and one 

 peculiar siliceous greenstone, the original source of which might pos- 

 sibly be identified. It is very improbable that any of them are 

 British, much more likely that they are Scandinavian. Possibly, 

 of course, they may be parts of ballast, but I confess it seems to me 

 much more probable that they are portions of Scandinavian drift. 

 The evidence is slight, but it seems the only kind obtainable, and it 

 may be taken for what it is worth. 



The mass of the beach along our south-eastern coasts, of course, 

 consists of Chalk flints, but even these are of a very varied character, 

 the majority being unaltered and referable at once to the original 

 Chalk, while others bear marks of having once belonged to Eocene 

 pebble beds (London Clay basement, Oldhaven or Bagshot), a few 

 to the Isle of Thanet Sands, and a very large proportion, as I infer 

 from their brown coatings and sub-angular forms, to glacial gravels 

 or drift. J. A. Birds. 



St. Leonakds-on-Sea, Oct. 16, 1880. 



ON THE DISTRIBUTION OF VOLCANOS. 



Sir, — Having been absent during the last summer in the north 

 part of Yezo and the Kurile Islands, it was not until a few days ago 

 that 1 received your Number of May, 1880, in which there is a 

 criticism of a short paper of mine on the " Geographical Distribu- 

 tion of Volcanos," published by you in April, 1880. 



This paper was chiefly written for the purpose of pointing out a 

 fact, which, so far as I am aware, had not previously been noticed, 

 namely, volcanos are chiefly distributed along the borders of laud 

 which slopes steeply beneath the sea. 



Whilst suggesting an explanation for this I had reason to refer to 

 the position of an isothermal surface lying partly under the land and 

 partly under the sea. I then said that it was not unlikely that 

 this surface would be found at a "much greater depth beneath the 

 rocks which form the bed of the ocean," than the depth at which we 

 should find it beneath the land. 



