346 ON THE MOVEMENTS OF THE EARTh's CRUST. 



Processes of plication may also perhaps be a consequence of tlie move- 

 ment of "tables" not being of the same kind on both sides. 



But the boundaries between the deep ocean and the foot of the con- 

 tinents do not everywhere coincide with the existing shore. Along the 

 coasts there are often shallow tracts in the sea. These are the foot of 

 the land which the sea has flooded, and the great deep sea only com- 

 mences farther out. 



The Trias period has received its name because it shows a distinct 

 triple division. It commences with fresh-water and littoral formations, 

 upon which follow formations of deeper water, and then closes with 

 fresh-water and shore formations. At its commencement the land was 

 high relatively to the sea ; as it went on the sea rose higher and higher ; 

 then the land again began to rise, and at the end of the period it was 

 again high in relation to the sea. And these great changes in the situ- 

 ation of the coastline were no doubt effected by means of many smaller 

 oscillations. 



But just as it is with the Trias, so is it also with other geological 

 formations. They commence with littoral formations (there is often a 

 conglomerate at the bottom) ; these are followed by deeper marine 

 formations, and at the close we have again shore formations. The name 

 of Trias would therefore really apply to all of them. The first person 

 to call attention to this remarkable triple division of formations would 

 seem to have been Eaton. It was subsequently discussed by J. S. New- 

 berry in his memoir entitled " Circles of Deposition in American Sedi- 

 mentary Kecks" {Proc. Amer. Assoc, 1873, vol. xxii, p. 185), and Hull 

 {Trans. Geol, Soc. Glasgow, 1868, ill., pt. 1, p. 39) ; see also A. Geikie, 

 " Text-book of Geology," p. 498, where further references to literature 

 will be found. Principal Dawson called these tripartite periods " cycles," 

 and in his "Story of the Earth and Man" he established the following 

 cycles of this kind : (1) Cambrian ; (2) Lower Silurian ; (3) Upper Silu- 

 rian ; (4) Devonian ; (5) Carboniferous ; (C) Permian ; (7) Trias ] (8) 

 Lower Jurassic ; (9) Middle Jurassic; (10) Upper Jurassic; (11) Creta- 

 ceous ; and (12) Tertiary.* It appears therefore that these cycles are 

 periods of long duration; each of them has certainly lasted several 

 hundred thousand years. And in the middle of each cycle the great 

 overflows of the sea have attained their highest point. The cycles 

 alternate with continental periods. During the elevation of the land 

 the horizontal position of the strata was often disturbed, so that the 

 deposits of the new cycle lie unconformably upon the older ones. 



In this way the development has gone on, at any rate in the northern 

 hemisphere. Mojsisowics, Suess, and others have pointed out that it 

 has taken place simultaneously in the same direction in Europe, Asia, 

 and Xorth America. These great changes have taken place over the 

 whole of the northern hemisphere, and on both sides of the oceans they 



We sball see hereafter that this formation includes two cycles. 



