Page:A Treatise on Geology, volume 1.djvu/265

 CHAP. VI.

To determine exactly the geological date of a disruption of the crust of the globe is not easy, even when the case is so simple as that of a common "fault;" when it is to apply to a whole chain of mountains no more difficult problem can be proposed to geological observers. In the present case it is rendered still more perplexing by the change of mineral and organic characters which, on the flanks of the Pyrenees, almost destroys the distinction of secondary and tertiary deposits, and leaves little relation between the Apennine limestone and the chalk of Northern Europe, except what the scaglia of Lombardy has afforded.

As far as regards the British islands, a gradual or interrupted rising of the whole bed of the sea would much better suit the phenomena than one mighty convulsion; and Mr. Lyells views of the gradual rising of the Weald, though, perhaps, not entirely satisfactory in that particular instance, contain an important illustration of the consequences of such an hypothesis.

Offering a most decided contrast with the secondary and older strata in most of their essential characters, the tertiary strata form a division of the series which may be considered as of more elevated rank than the term "system," in our mode of using it (which is now become common) denotes. But, on the other hand, so many analogies appear among these strata of all ages, that, though with great propriety distinguishable into "formations," they must, for the present at least, be ranked in one general system.