4 PRINCIPLES OF STRATIGRAPHY 



accompanying rise and transgression cause positive eustatic move- 

 ments. A third type of movement must be considered. If we re- 

 gard the continents as crustal blocks of a lighter specific gravity 

 than the suboceanic crustal blocks, the former being squeezed 

 upward by the downward pressure of the suboceanic blocks, it 

 is conceivable that a single continental block like that of the 

 Americas might be squeezed upward, while the other conti- 

 nental blocks would remain stationary, and yet at the same time 

 there need be no marked change in the altitude of the suboceanic 

 blocks with reference to the stationary continental blocks. As a 

 result there will be a widespread emergence, or negative move- 

 ment of the sea, so far as the rising land block is concerned, while 

 at the same time the resultant displacement of the water will 

 cause a transgressive or positive movement of the sea with refer- 

 ence to the stationary blocks. Thus a retreatal movement in one 

 continent may be correlated with a transgressive one in another 

 continent. Such differential movements may even affect different 

 parts of the same continental block, if the movement is an unequal 

 or tilting one, and this would account for transgression of the sea 

 over one section of a continent at a time when retreatal move- 

 ments characterized another part. 



The Terrestrial Part of the Hydrosphere. The terrestrial 

 or extra-oceanic part of the hydrosphere is chiefly found in the 

 ground water and in the streams, lakes and ponds of the earth's 

 surface. Since the porosity of the rocks constituting the earth's 

 surface region varies greatly, the quantitative distribution of the 

 water also varies. The ground water is probably chiefly confined 

 to the upper six miles of the earth's crust, and here we have a 

 diminishing porosity from 5 per cent, or over at the surface to 

 zero at the depth of six miles. It has been estimated on this basis 

 that the rock of the earth's surface contains enough water to form 

 a layer nearly 800 feet deep (Chamberlin and Salisbury -4 i-'i"/), 

 but other estimates make this layer much thicker, that of Slichter 

 being 3,000 to 3,500 feet (25:75). Van Hise, on the other hand, 

 has shown that the amount is much less, and is, according to his 

 estimate, sufficient to make a layer only 69 meters or 226 feet 

 thick over the continental area {t^O'.i28-I2(), 570- j/i). Kemp has 

 called attention to the fact that in the deep mines no water is en- 

 countered below a moderate depth. "In several important in- 

 stances of this class, as well as in many mines of smaller depth, 

 it is possible to impound all the water within a short distance, it 

 may be within 500 feet of the surface. Below this level the work- 

 ings are dry, and in not a few cases dusty." (i5:/(5.) He con- 



