768 J. BAREELL MEASUREMENTS OF GEOLOGIC TIME 



which were covered in the late Pleistocene by continental ice-sheets. 

 These shallow-water bodies appear to be, consequently, lingering effects 

 of the local depression of, the crust under the weight of the ice. 



Eliminating these special conditions, let the continents be contrasted 

 with their past states in amount of submergence, and it is seen that not 

 only is there a notable absence of internal floodings, but a very notable 

 elevation of the continents as a whole. 



The great contrast between the present broadly elevated lands and the 

 low and flooded condition of the continents during epochs of geologic 

 quiet marks out the later Tertiary and the Quaternary as together consti- 

 tuting a great period of revolution. This, as a general statement, is M^ell 

 appreciated, but certain of the logical consequences have not leavened 

 geologic thought. 



The ancient epeiric seas were typically very shallow-water bodies. The 

 evidence is clear and varied. The lime-depositing seas have formerly been 

 regarded as the deepest, yet bottom-growing algse were at times abundant, 

 as seen in the cryptozoan horizons. Sunlight must therefore have freely 

 penetrated to the bottom. But more emphatic testimony is given by inter- 

 formational conglomerates, which show during the deposition of the beds 

 a vigorous stirring of the bottom by wave action. Still more positive in 

 •meaning are the mud-cracks which are abundant in certain horizons. 

 These are not shore phenomena, since they may occur at the same horizon 

 over areas reaching thousands of square miles; neither are they of tidal 

 origin, as shown by their breadth and the absence of tidal channels. With 

 seasonal change of winds, or more probably with slight oscillations in 

 level of longer period, the water came and went. 



The wide-spread marine sands and silts of these seas show the effective- 

 ness of wave action in agitating the bottom material and working it by 

 oscillatory action to great distances. Over a broad reach of shallow water 

 the wave action is adjusted to depth, being damped down by the work of 

 stirring the bottom. Furthermore, in times of equable climates, although 

 the circulation of air and water was very effective in distributing the 

 equatorial warmth to high latitudes, yet the intensity of winds was prob- 

 ably less. Winds are due to differences of temperature and pressure and 

 are in general most intense where these conditions rise highest, as in 

 middle temperate latitudes in winter. The wave action in the epeiric 

 seas was then more comparable to that which now exists over broad, shal- 

 low lakes or bays in the warmer climates rather than comparable to the 

 waves of the deep and open oceans. 



From these lines of evidence depths of 20 and 50 to 100 feet may be 

 regaixled as typical. Occasional deeper waters must of course not be ex- 



