64 TEE POPULAR SCIENCE MONTHLY. 



in another, they have been constantly fluctuating. Hall and Dana 

 have well illustrated these points in so far as Eastern North America 

 is concerned. Professor Hull, of the Geological Survey of Ireland, has 

 recently had the boldness to reduce the fluctuations of land and water, 

 as evidenced in the British Islands, to the form of a series of maps 

 intended to show the physical geography of each successive period. 

 The attempt is probably premature, and has been met with much ad- 

 verse criticism ; but there can be no doubt that it has an element of 

 truth. When we attempt to calculate what could have been supplied 

 from the old eozoic nucleus by decay and aqueous erosion, and when 

 we take into account the greater local thickness of sediments toward 

 the present sea-basins, we can scarcely avoid the conclusion that ex- 

 tensive areas once occupied by high land are now under the sea. But 

 to ascertain the precise areas and position of these perished lands may 

 now be impossible. 



In point of fact, we are obliged to believe in the contemporaneous 

 existence in all geological periods, except perhaps the very oldest, of 

 three sorts of areas on the surface of the earth : 1. Oceanic areas of 

 deep sea, which must always have occupied the bed of the present 

 ocean, or parts of it ; 2. Continental plateaus, sometimes existing as 

 low flats or as higher table-lands, and sometimes submerged ; 3. Areas 

 of plication or folding, more especially along the borders of the oceans, 

 forming elevated lands rarely submerged, and constantly affording the 

 material of sedimentary accumulations. 



Every geologist knows the contention which has been occasioned 

 by the attempts to correlate the earlier palaeozoic deposits of the At- 

 lantic margin of North America with those forming at the same time 

 on the interior plateau, and with those of intervening lines of plication 

 and igneous disturbance. Stratigraphy, lithology, and fossils are all 

 more or less at fault in dealing with these questions ; and, while the 

 general nature of the problem is understood by many geologists, its 

 solution in particular cases is still a source of apparently endless de- 

 bate. 



The causes and mode of operation of the great movements of the 

 earth's crust which have produced mountains, plains, and table-lands, 

 are still involved in some mystery. One patent cause is the unequal 

 settling of the crust toward the center ; but it is not so generally un- 

 derstood as it should be that the greater settlement of the ocean-bed 

 has necessitated its pressure against the sides of the continents in the 

 same manner that a huge ice-floe crushes a ship or a pier. The geo- 

 logical map of North America shows this at a glance, and impresses 

 us with the fact that large portions of the earth's crust have not only 

 been folded, but bodily pushed back for great distances. On looking 

 at the extreme north, we see that the great Laurentian mass of cen- 

 tral Newfoundland has acted as a protecting pier to the space imme- 

 diately west of it, and has caused the Gulf of St. Lawrence to re- 



