112 Geology of Wiltshire. 
long line of coast running in a N.E. and 8.W. direction across this 
island, (the lower oolitic group.) Eastward of this coast and 
separated from it by a deep deposit of mud, was formed, ata 
later period, a long parallel range of coral reef, (the Oxford 
clay and Coral rag). These coralline ranges were, probably, for 
some time sinking below the level at which the insects formed them 
(as is believed to be the case at present with the coral reefs of the 
Pacific,) and were in part covered by another vast thickness of clayey 
sediment (the Kimmeridge clay,) and by other calcareous strata (the 
Portland beds;) after which a new elevation of this area must have 
occurred, bringing up the surface above the sea-level, so as to give 
rise to the formation of the Purbeck beds in fresh or brackish water. 
Another depression must have then taken place, during which the 
sands of the lower Cretaceous group accumulated, and which, pro- 
bably, reached its maximum in the period when the thick mass of 
white chalk was slowly and quietly settling at the bottom of a ‘deep 
deep sea.’ Then this district rose again so near the sea-level as to 
receive the littoral gravel, clays, and sands of the lower and middle 
Eocene groups. And the rise seems to have continued until these 
marine beds, with the chalk on which they rest, not only became 
dry land, but reached the elevation at which we now find them of 
1000 feet above the sea. 
That all these several changes occupied periods of immense dura- 
tion, is proved not merely by the enormous thickness of the various 
beds thus heaped one over the other, but still more by the changes 
of animal life which accompanied their deposition—thousands of 
species having become extinct one by one, and succeeded by other 
thousands as gradually brought into being ;—those found in the 
more recent or highest beds becoming by degrees more and more 
like those which are now in existence, until some few at first, and 
afterwards a large proportion, appear identical with them. By 
what secondary causes the changes in animal life were brought 
about, remains (and. will probably always remain,) a mystery. 
The same may be said, perhaps, of the contemporary changes of 
superficial level. Both processes were, undoubtedly, gradual and 
continuous through countless ages. And during all that time not 
