ees = ee Oe eee ee a oo a 
a ee 
__ 
— 
58 THE TESTIMONY OF THE ROCKS. 
surance that this layer of limestone or sandstone is of 
earlier, that, of later origin. As a matter of fact, the 
textbooks do treat the various “ages” of geology as if 
they corresponded to certain strata of the earth’s crust. 
But by what method is the age of the various layers de- 
termined? James D. Dana in his “Manual of Geology” 
(Fourth edition, p. 398 f.) says that there are four 
methods by which we may decide the relation of one 
laver to another. ‘The first is, naturally, the order in 
which the layers rest upon one another; the lower strata, 
are, of course, older than the upper. However, he points 
out in four “precautions” the inability of the investigator 
to depend on this method, since “for the comparing of 
rocks of disconnected regions, this criterion must fail.” 
Also the color and mineral composition can be used only 
“with distrust” and must be “usually disregarded.” Then 
the Manual proceeds: “4. Fossils—The criterion for 
determining the chronological order of strata dependent 
on kinds of fossils takes direct hold upon time, and, 
therefore, 1s the best; and, moreover, it serves for the 
correlation of rocks all over the world.” Now observe 
how, in the following, the geologist leans upon the evo- 
lutionist: “The life of the globe has changed with the 
progress of time. Each epoch has had its peculiar spe- 
cies, or peculiar groups of species. Moreover, the suc- 
cession of life has followed a grand law of progress, in- 
volving under a single system a closer and closer ap- 
proximation in the species, as time moved on, to those 
which now exist. It follows, therefore, that identity of 
species of fossils proves approximate identity of age.” 
Let us bear this in mind. Dana takes for granted the 
evolutionary process. The simpler forms of animal life 
indicate the older strata, the complex forms, the more 
recent. We do not misunderstand Mr. Dana. Such ex- 
