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Prof. J. Prestwich. 



yielding of the crust during this late geological epoch been confined 

 to these limits. Great parts of India, Australia, and of the seaboard 

 of North and South America, have been under the sea and raised at 

 various intervals during the whole of the Tertiary period. 



But it is not these movements — great and general as they were — 

 that so immediately concern the question before us. Our object is to 

 show that the flexibility of the crust, which is exhibited in all geolo- 

 gical periods, has been continued without break and over large areas 

 down to the latest period, and that the older changes link on to 

 changes in progress in our own times — changes admitting of a 

 measurement which enables us to realise their importance. 



The presence of shells of recent species at certain elevations in 

 Central England and Wales prove that those areas have undergone 

 an elevation of not less than 1400 to 1500 feet after the inset of the 

 Glacial period. Ireland and Scotland have undergone changes little 

 less in the same period ; and so likewise has the North American 

 continent. The rock of Gibraltar has been raised 1400 feet or 

 more at even a later period. The massif of Scandinavia, the long 

 coast-lines of the Pacific side of South America, have been raised 

 500 or 600 feet or more during the life of existing species of 

 Mollusca. Over portions of the Pacific basin islands have been raised 

 200 to 300 feet or more, and the coasts of Arctic America and Asia 

 exhibit conclusive evidence of similar recent elevation. These move- 

 ments of elevation, which bring us down to the threshold of the 

 present times, link on without break in the latter areas (and many 

 others might be named), with the changes of level which some of those 

 areas are still undergoing. 



Changes of level have been ascribed by some to the change of tem- 

 perature at depths, caused by the shifting upwards of the underground 

 isotherms, in proportion as the strata have increased in thickness by 

 the successive addition of fresh sediment. This, as suggested by 

 Babbage and Herschel, may be a true cause under certain conditions 

 of thick accumulation of strata, but must necessarily, as the expan- 

 sion of rocks by heat is so small, be limited to moderate vertical dis- 

 placement, and fails to explain the greater changes of level to which 

 we have just referred, for neither in the Old nor in the New World 

 do the later Tertiary strata exceed, as a rule, a thickness of 1000 to 

 3000 or 4000 feet, so that the difference of temperature in the strata 

 so covered up would not usually exceed 20° to 100° F., and would 

 rarely attain to a rise 150° F. Supposing this to affect an underlying 

 mass of rock, say 100,000 feet in depth, the effect would be to increase 

 its dimensions vertically from about 10 to 100 feet only. This also is 

 on the assumption that the underlying mass of rock remains stationary ; 

 but if, whilst the accumulation of strata proceeded, the area itself, as 

 has usually been the case, subsided, the only effect of expansion of the 



