The modern form of the theory substitutes onion-coats of
fossiliferous rocks for onion-coats of mineral and lithological
characters; and a brief consideration of this theory is now in order.
About the time that various geologists here and there were finding rocks
in positions that could not be explained in terms of Werner's theory,
William Smith (1769-1839) in England and the great Baron Cuvier
(1769-1832) in France found characteristic fossils occurring in various
strata; and under their teachings it was not long before the fossils
were considered the best guide in determining the relative sequence of
the rocks. The familiar idea of world-enveloping strata as representing
successive ages was not discarded; but instead of Werner's successive
ages of limestone making, sandstone making, etc., these new
investigators taught that there were successive ages of invertebrates,
fishes, reptiles, and mammals, these creatures having registered their
existence in rocky strata which thus by hypothesis completely encircled
the globe one outside another.
It is true that early in the nineteenth century Sir Charles Lyell and
others tried to disclaim this absurd and unscientific inheritance from
Werner's onion-coats; but modern geology has never yet got rid of its
essential and its chief characteristic idea, for all our text-books
still speak of various successive ages _when only certain types of life
prevailed all over the globe_.
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