548 



dred yards forming their base. The contact of the lias filled -with 

 Belemnites with the inferior oolite is here marked by a highly ferrugi- 

 nous oolitic bed, only a few inches in thickness, but containing almost 

 as many fossils as when of its greatest thickness. These are gene- 

 rally identical with those found at Dundry. Upon this bed rests the 

 sandy calcareous rock, 25 feet thick, which has been described by 

 M. De la Beche as lias. It contains three or four beds full of sponges, 

 Alcynia, a few shells, and numerous Echini, and is surmounted by 

 an argillaceous deposit varying from light grey to dark blue, and as- 

 suming at Port en Bessin a thickness of more than 100 feet, but is 

 not separately distinguished in M. De la Beche's section. From geo- 

 logical position it appears to represent the Fullers' earth. This clay 

 is covered by a calcareous rock slightly oolitic, containing few or no 

 fossils, and forming the summit of the cliffs from St. Coine to Gran- 

 ville. 



Mr. Pratt concludes his paper by observing that nearly the whole 

 of the strata found between the chalk and the lias in England are 

 found on the coast of Normandy, though somewhat modified ; and 

 that nearly all the characteristic shells are found in each. No bed 

 of any consequence found in Normandy is wanting in the English 

 series ; while the Portland and perhaps the Purbeck beds with the 

 Kelloway rock are not seen in this part of France. 



Extracts from a letter from Sir John F. W. Herschell to C. Lyell, 

 Esq., dated Fredhausen, Cape of Good Hope, 20th February, 1836, 

 were then read. 



The author commences by inquiring, whether it had ever occurred 

 to Mr. Lyell to speculate on the probable transfer of pressure from 

 one part to another of the earth's surface by the degradation of exist- 

 ing, and the formation of new continents, on the fluid or semifluid 

 matter beneath the outer crust ? Supposing the whole to float on a 

 sea of lava, the effect would merely be an almost infinitely minute 

 flexure of the strata ; but supposing the layer next below the crust 

 to be partly solid and partly fluid, and composed of a mixture of fixed 

 rock, liquid lava and other masses in various degrees of viscidity 

 and mobility ; great inequalities may subsist in the distribution of 

 pressure, and the consequences may be local disruptions of the crust 

 where weakest, and escape to the surface of lava, &c. 



Referring to the phenomena of volcanos, Sir J. Herschell observes, 

 that it has always been his greatest difficulty in geology to find a 

 primum mobile for the volcano, taken as a general and not as a local 

 phenomenon ; and referring to the different theories given on the 

 subject, which he considers insufficient, wanting in explicitness and 

 as not going high enough in the inquiry or up to its true beginning, 

 and also as giving in some respects a wrong notion of the process 

 itself; — inquires, how came the gases which are evolved to be con- 

 densed ? — why did they submit to be urged into liquefaction ? — if 

 they were not originally elastic, but have become so by subterranean 

 heat — whence came the heat, and why did it come ? — how came 

 the pressure to be removed, or what caused the crack ? 



It seems clear that if the gases or aqueous vapour were once free 



