Life Progressive. 405 
of its predecessor in time. Though it is apparent that there 
has not been continuity in any given area, still the geological 
chain could not have been snapped at one point and taken 
up again at a totally different one. Hence we arrive at the 
conviction that in geology, as in other sciences, continuity is 
the fundamental law, and that the lines of demarcation 
between the great formations are but gaps in our own 
knowledge. 
Through the study of fossils, as is well known, geologists 
have been led to the all-important generalization that the 
vast series of fossiliferous or sedimentary rocks may be 
separated into a number of definite groups or formations, 
each of which being characterized by its own organic 
remains, but not properly and strictly, it must be understood, 
by the occurrence therein of any one particular fossil. 
However, a formation may contain some particular fossil or 
fossils not occurring outside of that formation, thus enabling 
an observer to identify a given group with tolerable certainty ; 
or, as very often happens, some particular stratum or sub- 
group of a series, may contain peculiar fossils, whereby its 
existence may be determined with considerable readiness in 
divers localities. Each great formation, let it be said, is 
properly characterized by the association of certain fossils, 
the predominance of certain families or orders, or by an 
assemblage of fossil remains that represent the life of the 
period during which the formation was deposited. 
Fossils, then, not only enable us to determine the age of 
the deposits in which they are found, but they also further 
enable us to arrive at some very important conclusions 
respecting the manner in which the fossiliferous bed was 
deposited, and, consequently, to the condition of the par- 
ticular region occupied by the bed at the period of its 
formation. Beds that contain the remains of animals, such 
as now inhabit rivers, we know to be fluviatile in their origin, 
and that at one time they must have either constituted 
actual river-beds, or been deposited by the overflowing of 
