984 PRINCIPLES OF STRATIGRAPHY 



which are the inland cutting of waves and currents, thus extend- 

 ing the sea landward ; and the deposition of the land-derived detri- 

 tus on the edge of the continental shelf, thus pushing this edge 

 seaward and widening the submarine platform. Subsidence of 

 the land permits the advance of the sea over the low country, and 

 broad epicontinental seas are formed, which fall entirely within 

 the littoral life district. In mediterraneans, too, the littoral Ijelt is 

 generally a broad one, only the central portion descending to abyssal 

 depth. (See further, Chapter III.) 



The conditions become more favorable for the development of 

 littoral life as the continent grows older, provided, of course, that 

 no important oscillatory movements occur. As time progresses the 

 breadth of the submerged continental shelf increases, both by land- 

 ward cutting and by seaward building, and the surface of the land 

 becomes more and more reduced, thus causing a decrease in the 

 amount of detrital material which can be carried into the sea, and 

 a concomitant increase in the purity of the water. When peneplana- 

 tion, or the reduction of the land to base-level, has been accom- 

 plished, the amount of detritus carried into the sea is practically 

 nil; and any organisms, like corals, adapted only to pure water, 

 can flourish close to the continental shore, and deposits of a purely 

 organic nature, such as extensive deposits of pteropod or foramini- 

 feral shells, may form in comparatively shallow water. 



The epicontinental sea and the littoral belt of the mediterraneans 

 are especially adapted for the development of local or provincial 

 faunas. Such provincializing of faunas is most marked if, by some 

 oscillatory movement of the land or some other physical change, 

 the basins of these intracontinental seas become separated from the 

 littoral district of the ocean sufficiently to prevent intercommunica- 

 tion between the organisms of the two provinces. A barrier is thus 

 formed, which need not necessarily be a land barrier, and a great 

 diversity of faunas may result. A rather marked diversity of fauna 

 existed in early Tertiary time between the Mississippi embayment 

 and the Atlantic coast, and in Palzeozoic time between the Bay of 

 New York and the central interior sea. Recent provincial faunas 

 are frequently met with. It requires only a comparatively slight 

 elevation of the sea-floor, or a moderate deepening of the abyssal 

 oceanic basins, to draw off the water from the shallower regions 

 and lay dry large portions of the littoral district. Such a change 

 would, of course, result in an almost complete extinction of the 

 littoral flora and fauna thus exposed and force the survivors to ac- 

 commodate themselves to a narrower field. Revival of stream ac- 

 tivities, consequent upon elevation of the land, would result in 



