118 Prof. W. Thomson on the Depths of the Sea. 



formed during the periods of depression, and now exposed by 

 an upheaval of this minor character, are all comparatively 

 local and shallow-water beds, as shown by the nature and 

 the richness of their faunae. To put this in another form : 

 there is no reason to suppose that either the physical or the 

 biological conditions of two-thirds of the ocean have been 

 affected by the oscillations which produced the varying distri- 

 bution of the sea and land and the local modifications and 

 migrations of faunse during the Tertiary period. No doubt 

 the temperature of the different portions of the deep sea has 

 altered again and again, owing to geographical changes in- 

 fluencing the distribution of the minor currents and the branches 

 of the great currents ; and it is to the accumulation of these 

 slight changes through countless ages that we must look as 

 the cause of the gradual modification of the fauna of the chalk, 

 of the extinction of some animal groups, and the greater deve- 

 lopment of others. A bit of the edge of the Cretaceous forma- 

 tion has been tilted up, to form the white cliffs of Albion and 

 the chalk-beds of France ; but the great mass of the formation 

 maintains nearly the same character, and is now entombing 

 the same group of organisms, among the Philippines, off the 

 coast of Spain, in the seas of Japan, near the coast of Massa- 

 chusetts, off the Faroes, and to the extreme Lofoden Islands. 

 I imagine that this is one of the great formations — one of the 

 corner-stones in the building of the earth, formed slowly in 

 vast areas of subsidence, which will only make its appearance 

 in mass along with a complete change in the distribution of 

 land and sea, and which may be expected in some places to 

 resist denudation, and to stand like the mountain-limestone, 

 as one of the odd pages of a future geological record. Some 

 great peculiarities in the distribution of the Miocene land flora 

 have led to the idea that one of these minor oscillations may 

 have depressed the " telegraph plateau " during later Tertiary 

 times. It may be so, though I think the evidence is very 

 unsatisfactory ; but it is by no means necessary that every 

 part of the present cretaceous basin should have been sea 

 throughout ; whenever it was sea, however, it was continuous 

 in space with a sea which had been continuous in time (pro- 

 bably, at all events, from the commencement of the Jurassic 

 period) , and was peopled from that sea. If these views prove 

 correct, they must modify considerably our interpretation of 

 geological history. 



Chalk-flints. — There is one point in the structure and com- 

 position of the white chalk which distinguishes it, in the most 

 marked way, from the modern deposits of the Atlantic. Modern 

 soundings and dredgings from all depths are full of delicate sili- 



