[ 487 1 



LVII. On the probable Cause of the Displacement of Shore- 

 lines, an Attempt at a Geological Chronology. By A. 

 Blytt. 



[Plate X.] 



[Concluded from p. 429.] 



THE Trias period has received its name because it shows a 

 distinct triple division. It commences with freshwater 

 and Httoral formations, upon which follow formations of 

 deeper water, and then closes with freshwater and shore- 

 i'ormatious. At its commencement the land was high re- 

 latively 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 situation of the coast-line 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 forma- 

 tions (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 •'ipply fo 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 dis- 

 cussed by J. S. Newberry in his memoir entitled " Circles of 

 Deposition in American Sedimentary Rocks " (Proc. Amer. 

 Assoc. 1873, vol. xxii. p. 185), and Hull (Trans. Geol. Soc. 

 Glasgow, 1868, iii. pt. 1, p. 39) ; see also A. Geikie, ' Text- 

 book of Geology,' p. 498, where further references to litera- 

 ture 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 Silurian ; 4. Devonian ; 

 5. Carboniferous ; 6. Permian ; 7, Trias ; 8. Lower Jurassic ; 

 9. Middle Jurassic ; 10. Upper Jurassic ; 11. (Cretaceous ; 

 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 

 * We shall see hereafter that this formation includes two cycles. 



