2 HISTORICAL GEOLOGY 



3. All, or nearly all, of the surface of the lithosphere has at some 

 time, or times, been covered by sea water. Stratified rocks of marine 

 origin now constitute fully five-sixths of the exposed surface of 

 the lithosphere, and it is certain that from most, at least, of the 

 remaining surface such stratified rocks have been removed by- 

 erosion. 



4. The continents were roughly outlined in early geologic time. 

 This is proved by the facts that even the oldest known rocks con- 

 tain much land-derived sediment of comparatively shallow water 

 origin and that there are no deposits which show that great oceanic 

 abysses ever extended across what are now continental areas. 

 Much evidence points to a very early development of oceanic 

 basins and continental masses which have occupied essentially the 

 same positions to the present time. 



5. During geologic time there has been a progressive tendency 

 for the continental masses to become higher and broader. There have 

 been many oscillations of level, accompanied by transgressions or 

 retrogressions of the sea, but the processes of elevation (relatively 

 speaking) have been predominant, while, at the same time, the sea 

 bottoms have become narrower and deeper. Just prior to the last 

 Ice age, and only a short time ago, geologically considered, North 

 America was even higher and broader than at present, for then the 

 shallow marginal sea bottoms (continental shelves) were also dry 

 land. 



Organic Inferences. 1 — 1. Organisms inhabited the earth many 

 millions of years ago. All but possibly the very oldest known series 

 of rocks contain organic remains. 



2. Throughout the known history of the earth organisms have 

 continuously changed. Each epoch of earth history or series of 

 strata has its characteristic assemblage of animals and plants. 

 The more ancient strata contain no species like those living to- 

 day, the latter being found only in rocks of comparatively (geo- 

 logically) recent date. Further, " the organisms which inhabited 

 the earth during any geological epoch were descended from organ- 

 isms of preceding epochs" (W. H. Norton). 



3. The change in organisms has been progressive. In early 

 geological time the animals and plants were comparatively simple 

 and low in the scale of organization and structure, and through the 



1 These statements of organic inferences follow, in the main, Norton's 

 Elements of Geology. 



