

FOSSILS 103 
contained fossils having the same general facies—and 
especially if several species were common to the limestones, 
the shales, and the sandstones, we could no longer doubt 
that all these rocks were accumulations formed in one and 
the same sea. Fossils are thus of paramount importance in 
the correlation of strata. 
In the attempt to determine the relative age of our 
fossiliferous strata, the most important step was taken when 
William Smith, the father of Stratigraphical Geology, deter- 
mined the sequence of the Mesozoic strata of England, and 
ascertained that each subdivision of that great series of rocks 
was characterised by the presence of certain particular types 
of fossils, Following his lead, geologists have since estab- 
lished the stratigraphical succession of fossiliferous strata 
throughout the major portion of the world. It is now 
recognised that every well-defined formation is marked by 
the presence of a particular flora or fauna, or by certain 
genera and species which are restricted to it. The presence 
of these type-fossz/s, as they are termed, enables the geologist 
at once to assign the rocks in which they occur to some 
definite horizon or stage in the great succession of sedi- 
mentary accumulations. 
It is obvious, therefore, that some knowledge of type- 
fossils must be of great use in Practical Geology. How 
greatly they help a geologist in his endeavour to work out 
a stratigraphical succession will be shown when the subject of 
seological surveying comes to be discussed. 
