283 
Geikie states further (p. 1146) that a considerable stratigraphical and 
paleontological break is to be remarked at the line between the Portlandian 
and the Purbeckian. Chamberlin and Salisbury (Geology, ii, p. 639) tell 
us that the close of the Paleozoic was marked by much more considerable 
geographic changes than the close of any pericd 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. 
e 
4, THE PRINCIPAL DIVISIONS OF GEOLOGICAL History ARE BASED ON Fossin 
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 quote 
from Lapparent (Traité de Géologie, ed. 5, p. 717): 
Il résulte de ces diverses considérations que les seules ressources de la 
stratigraphie, si précieuses et si indispensables qu’elles puissent @tre, sont 
insuffisantes pour l’établissement des grandes divisions de la géologie. I] 
faut done recourir a quelqu’argument d’une portée plus générale. Cet 
argument, nous allons le trouver dans la considération des faunes et des 
flores 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, erosional! 
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 hua- 
man, 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 
