100 ROCK-METAMORPHISM. 



during the period of formation of any one system, a cycle of 

 vertical movements, upward and downward, which alternated 

 with those of another cycle in an adjacent region*. Thus, as- 

 suming a given region to be affected by an upward movement, 

 it would be in alternating correlation with an adjacent region, 

 where the opposite or downward movement would be going on. 

 These movements would necessarily cause a region to be occu- 

 pied by land, say in the beginning and ending of a given systemal 

 period f, and by water in the middle division of the same 

 chronological term. 



It will also be understood that the maximum of the down- 

 ward movement of any one period would submerge a region to 

 its greatest depth beneath the level of the sea; while the 

 maximum of the upward movement would place it, as land, at 

 its greatest altitude. 



Taking the vertical movements of a single cycle, the region 

 affected by them would present a succession of land- and sea- 

 features representing their different stages of development. 

 Thus, in a region which has attained its maximum elevation, and 

 which it may be assumed is in the 1st stage, land interspersed 

 with lakes, rivers, and bordered by estuaries would predominate, 

 producing lacustrine and estuarine marls, clays, and sands ; also 

 other deposits, often terrestrial or conglomeratic, all more or 

 less charged with the remains of a fauna and flora bespeaking the 

 prevalence of widely-spread terrestrial conditions. In the 2nd 

 stage, the movement being downward, and the sea necessarily 

 encroaching on the lowlands, marine, littoral, and shallow-water 

 deposits argillaceous, sandy, marly, and calcareous would be 

 thrown down. In the 3rd stage that in which the downward 

 movement reached its maximum, and when the sea (Thalassa) 



* An abstract of a paper containing this hypothesis appears in the ( Pro- 

 ceedings of the Koyal Irish Academy/ ser. 2, vol. iii. (Science), No. 5, Dec. 

 1880. A region is assumed to be of continental extent. See Supplementary 

 Note A. 



t By a " systemal period " I mean a division of geological time, during 

 which a rock-system was in process of formation : it may be said to corre- 

 spond to a single cycle of vertical movements, Taking the masses of deposit 

 constituting a rock-system, the mutations of its life-features, and the suc- 

 cessive physical phenomena it witnessed, the conviction is strong in me 

 that a systemal period is so vast as to be utterly beyond approximately 

 calculating. 



