DYNAMICAL GEOLOGY. 293 



with variations in the earth's rotatory force, and consequently 

 in the length of day and night, or with any incongruity between 

 the earth's centre of gravity and the centre of form. 



Professor Penck agrees with Suess in the leading principle 

 that secular variations are due, not to crust-movements, but to 

 fluctuations of sea-level. He doubts, however, the possibility 

 of the equilibrium between land and water being disturbed by 

 general variations of the earth's gravity. He traces all changes 

 of level in maritime tracts of land to local re-distribution of 

 rock-material and consequent local alteration in the attractive 

 force exerted by the land upon the water-surface. Re-dis- 

 tribution may be produced by crust-folding, by the denudation 

 of adjoining continental areas by the sedimentation of organic 

 and inorganic deposits on the sea-floor, and most of all, in 

 Professor Penck's opinion, by the piling-up of colossal masses 

 of ice in particular regions. The American geologist, Mr. 

 Upham, has arrived, on independent grounds, at similar 

 conceptions of variation in the sea-level, although he at the 

 same time believes in the actual upheaval of land areas. 



The whole question is again discussed by Suess in the 

 second volume of his work, Das Antlitz der Erde. This 

 volume describes and compares the coast-line of the Atlantic 

 and Pacific Oceans, passes in review the distribution of the 

 oceans in all past geological epochs, and gives a complete 

 account of all relative changes of level between land and water 

 within historic time. The many sources of error and the 

 insufficiency of data are noted; and the several causes which 

 might have influenced the surface of the ocean are carefully 

 elucidated. Professor Suess adheres firmly to his view that 

 secular movements of elevation of land have been without 

 significance in determining the grander forms of the earth's 

 surface, and take place at the present day very exceptionally, 

 and only as local phenomena. He depicts a shrinking crust 

 or lithosphere, which as it contracts carries with it the immense 

 body of water on its surface. According to Suess, episodal 

 crust-subsidences have determined the form and position of the 

 ocean-basins at different epochs of the earth's history, and 

 have been accompanied by the corresponding widely-extended 

 negative movements of the ocean. The existing oceans repre- 

 sent areas whose subsidence may have occurred in various ages, 

 and whose boundaries are marked by lines of crust-fracture. 

 Bearing in view the vast extent and the uniformity of those 



