152 NEWER PLIOCENE STRATA. [Ch. XIII 



CHAPTER XIII. 



NEWER PLIOCENE STRATA AND CAVERN DEPOSITS. 



Chronological classification of Pleistocene formations, "why difficult — Freshwater 

 deposits in valley of Thames — In Norfolk cliffs — In Patagonia — Comparative 

 longevity of species in the mammalia and testacea — Fluvio-marine crag of 

 Norwich — Newer Pliocene strata of Sicily — Limestone of great thickness and 

 elevation — Alternation of marine and volcanic formations — Proofs of slow accu- 

 mulation — Great geographical changes in Sicily since the living fauna and flora 

 began to exist — Osseous breccias and cavern deposits — Sicily — Kirkdale — 

 Origin of stalactite — Australian cave-breccias — Geographical relationship of the 

 provinces of living vertebrata and those of the fossil species of the Pliocene 

 periods — Extinct struthious birds of New Zealand — Teeth of fossil quadrupeds. 



Having in the last chapter treated of the boulder formation and its 

 associated freshwater and marine strata as belonging chiefly to the close 

 of the Newer Pliocene period, Ave may now proceed to other deposits of 

 the same or nearly the same age. It should, however, be stated that it 

 is difficult to draw the line of separation between these modern forma- 

 tions, especially when we are called upon to compare deposits of marine 

 and freshwater origin, or these again with the ossiferous contents of 

 caverns. 



If as often as the carcasses of quadrupeds were buried in alluvium 

 during floods, or mired in swamps, or imbedded in lacustrine strata, a 

 stream of lava had descended and preserved the alluvial or freshwater 

 deposits, as frequently happened in Auvergne (see above, p. 80), keeping 

 them free from intermixture with strata subsequently formed, then indeed 

 the task of arranging chronologically the whole series of mammaliferous 

 formations might have been easy, even though many species were 

 common to several successive groups. But "when there have been 

 oscillations in the levels of the land, accompanied by the widening and" 

 deepening of valleys at more than one period, — when the same surface 

 has sometimes been submerged beneath the sea, after supporting forests 

 and land quadrupeds, and then raised again, and subject during each 

 change of level to sedimentary deposition and partial denudation, — and 

 when the drifting of ice by marine currents or by rivers, during an epoch 

 of intense cold, has for a season interfered with the ordinary mode of 

 transport, or with the geographical range of species, we cannot hope 

 speedily to extricate ourselves from the confusion in which the classifica- 

 tion of these Pleistocene formations is involved. 



At several points in the valley of the Thames, remnants of ancient 

 fluviatile deposits occur, which may differ considerably in age, although 

 the imbedded land and freshwater shells in each are of recent species. 

 At Brentford, for example, the bones of the Siberian Mammoth, or 



