T. Hart — Volcanic JExpIosious. 121 



clay that one "apparently passes into the other." There is no 

 evidence to suggest that the brickearth and gravel are newer than the 

 main mass of Boulder-clay, but it is possible there may be lenticular 

 inasses of Boulder-clay in the brickearths and gravel, as is known, 

 to be the case in the neighbourhood of Hertford. Clear sections, 

 however, were wanting in the neighbourhood of St. Albans, w^here 

 I found it difficult to draw a boundai-y-line between the brickearth 

 and the occasional remnants of Boulder-clay. In any case the 

 evidence strongly favours the view that the gravels and brickearths 

 (with Greywethers) belong to the Glacial period, and presumably 

 to that portion of it to which the " Middle Glacial " deposits of East 

 Anglia are assigned. Last year I saw at the Ipswich Museum a 

 large block of Hertfordshire pudding-stone, which Dr. J. E. Taylor 

 had procured from the Middle Glacial Sands of that neighbourhood. 

 It was a block measuring 6 feet x 4 feet x 1 foot 4 inches, and 

 weighing about 1-| tons. Floating-ice, in the shape of an iceberg, 

 was, in Dr. Taylor's opinion, the means by which this block had 

 been borne to its place of entombment. 



The conclusion suggested by these facts is that, while the pudding- 

 stone and Greywethers originated mainly in the Reading Beds and 

 in the Bagshot Sands, many blocks were incorporated in the Drift 

 during the Glacial period. 



To the Glacial deposits, the Thames Valley gravel owes the 

 greater portion of its constituents, the gravel was ready-made, and 

 in pursuing its course and widening its bounds the river ever and 

 anon may have torn away an accumulation in which one of the 

 Greywethers was imbedded. The evidence, however, justifies no 

 positive assertion with regard to the Bays water Grey wether. The 

 nearest remnants of Glacial Drift are six miles distant ; and it is 

 quite possible that this particular block may have been derived 

 directly from an outlier of Bagshot Sands, or it may have been left 

 as a relic of Pre-glacial denudation near the spot where it has now 

 been found. 



YII. — Notes on Volcanic Paroxysmal Explosions, and the 



Causes op Volcanic Action.^ 



By Thomas Hart, F.G.S. 



SINCE visiting the Volcanic region of Central and Southern Italy 

 in the autumn of last year, I have been much exercised from 

 time to time as to some of the conditions which produce, and sustain 

 volcanic activity. It seems to be an undoubted fact that water 

 coming into contact with highly heated rock is one of the most 

 important requisites to produce, and sustain a volcanic eruption. 

 The difficulty has been to explain how water is introduced. 



In this connexion I would mention that the principal active 

 volcanoes of the world are in close proximity to coast-lines, as 

 Chimborazo, Vesuvius, or inland areas surrounded by the sea, as 

 Hecla, Etna, and in New Zealand ; also in a more special degree in 

 such island groups as the Lipari, Krakatoa. 



^ Eead at the British Association, Section C (Geology), Leeds, 1890. 



