THE USES OF FOSSILS 397 
geological time-scale the rock belongs. With our pres- 
ent knowledge, we are able to determine the general age 
of any fossil-bearing rock found in any part of the earth. 
Early Attempts at a Division of the Strata. — Before the value 
of fossils was known, it was recognized that there were differ- 
ences in the age of rocks, and rough attempts were made to 
classify them. Since then the geological time-scale has gradually 
erown to its present condition. The names for the divisions fur- 
nish a crude record of this growth of ideas, and the time-scale 
thus contains a partial history of the changes in opinion. 
At one time rocks were divided into three classes, — Primary, 
Secondary, and Alluvial,—the Primary being the oldest, the 
Secondary, later rocks of sedimentary origin, and the Alluvial 
representing the unconsolidated surface rocks. Later these were 
divided into other classes, giving us Primitive, Transition, Sec- 
ondary, Tertiary,! and Alluvial. After this the belief that rocks 
of different ages were characterized by some mineral peculiarity, 
led to the introduction of other divisional names, such as the 
Cretaceous or chalk formation, the Old Red Sandstone, the Car- 
boniferous, ete. Still later, other names were introduced, or sub- 
stituted for some already in use, and these were based upon the 
study of fossils. In some cases these divisions were named for 
the place in which the study was made, and where the rocks were 
well developed, —as Devonian, for Devonshire, England, and 
Permian, for the province of Perm in Russia. 
Basis of the Geological Time-Scale. — The basis for 
the present division of the rocks into groups, is the 
fossils that they contain. The ¢rue life progress has 
been gradual and complete, without any abrupt break 
on the basis of which we can draw sharp lines of divi- 
1A name still used. 
