283 



Geikie states further (p. 114<>) that a considerable stratigraphical and 

 paleontological break is to be remarked at the line between the Portlandian 

 and the Purbeckian. Ohamberlin and Salisbury (Geology, ii, p. 639) tell 

 us that the close of the Paleozoic was marked by much more considerable 

 geographic changes thau the close of any period since the Algonkian. 

 The statement is qualified by the remark that these changes may be said 

 to have been in progress during the Permian rather than to have occurred 

 at its close. 



4. The Principal Divisions of Geological History Are Based on Fossil 



Organisms. 



It may therefore be confidently affirmed that the primary divisions 

 of geological history, as this history is now understood, are not based on 

 unconformities and deformations, great or small, between the successive 

 formations, but they are based on the history of the plants and animals 

 whose remains have become entombed in the rocks. I will here cpiote 

 from Lapparent (Ti-aite de Geologie, ed. 5, p. 717) : 



II resulte de ces diverses considerations que les seules ressources de la 

 stratigraphie, si precieuses et si indispensables qu'elles puissent etre, sont 

 insuffisantes pour l'etablissement des grandes divisions de la geologie. II 

 faut done recourir a. quelqu' argument d'une portee plus generale. Cet 

 argument, nous allons le trouver dans la consideration des faunes et des 

 iiores fossiles. 



It must not be supposed that the writer wishes to underestimate the 

 value to the geologist of changes in the materials that constitute suc- 

 cessive beds, of deformations of surfaces, or of unconformities, erosionai 

 and angular. All these indicate the physical changes that the earth was 

 undergoing and mark the subordinate and more or less local divisions of 

 geological history. Naturally the geologist in the field searches for such 

 interruptions in the course of deposition and, following a bent very ba- 

 nian, he may come to attach somewhat undue importance to them. In 

 any case, however, final recourse must be had to the fossils enclosed in 

 the rocks. Fossils are. to use a figure, the sands that, from the hour- 

 glass of the universe, have in an uninterrupted stream dropped into the 

 successive strata to mark the passage of time. Local interruptions of 

 sedimentation enable us to note the changes undergone by the organisms 

 that then existed; but whether there were breaks in deposition or not, the 



