EAKLY FIELD-WORK. 407 



volcanic vent is due to the expansion of water- vapour 

 contained within the molten rock under great pressure 

 and at a high temperature. But he had formed no 

 original conception of volcanic energy. Following Mr 

 Osmond Fisher's reasoning, he supposed that a thin 

 terrestrial crust rests on a slowly yielding viscous 

 layer within which lies a solid nucleus. The aqueous 

 vapour in volcanic eruptions he regarded as due to the 

 surface and underground waters with which the in- 

 tensely hot magma of the interior comes in contact, 

 and he believed that the actual cause of the uprise of 

 molten material and the outflow of lava is to be sought 

 in the effects of the secular refrigeration and contrac- 

 tion of this planet, the cooling and shrinking outer 

 shell compressing and forcing out the intensely heated 

 material inside. 



It is interesting to note that Prestwich began his 

 geological career by studying in minute and patient 

 detail the coal-field of Coalbrookdale, and that he was 

 thereafter led to explore the Old Red Sandstone of the 

 Moray Firth. This early work was so eclipsed by the 

 brilliance of his later researches among much younger 

 formations, that a later generation of his contem- 

 poraries hardly realised the rare excellence and orig- 

 inality of his first great essay. The elaborate memoir 

 on Coalbrookdale [7], presented to the Geological 

 Society when its author was only a young man of 

 twenty, is certainly a remarkable performance. Those 

 to whom it was first addressed can hardly have failed 

 to recognise in its author one of the future leaders of 

 English geology. Selecting an area of about 100 

 square miles, he carefully mapped its geology on 

 the scale of one inch to a mile. The map was no 

 mere sketch, but an elaborate survey, wherein the out- 



