. Elements of geology : a text-book for colleges and for the general reader. Geology. 410 PALEOZOIC SYSTEM OF ROCKS. on the Carboniferous; and, corresponding with this unconformity, there is a wonderful change in fauna and flora—a change the greatness of which we have attempted to show in the contrast on the preceding page. Now, the older geologists regarded this change as one of instan- taneous destruction and recreation, because they took no account of a lost interval. But we have already shown (pp. 181, 295) that in all cases of unconformity there is such a lost interval, which in some case
. Elements of geology : a text-book for colleges and for the general reader. Geology. 410 PALEOZOIC SYSTEM OF ROCKS. on the Carboniferous; and, corresponding with this unconformity, there is a wonderful change in fauna and flora—a change the greatness of which we have attempted to show in the contrast on the preceding page. Now, the older geologists regarded this change as one of instan- taneous destruction and recreation, because they took no account of a lost interval. But we have already shown (pp. 181, 295) that in all cases of unconformity there is such a lost interval, which in some cases is very large. In order to account for the very great change in the organic world, it is only necessary to suppose that periods represented by general unconformity are critical periods in the earth's history— periods of rapid change in physical geography, climate, and therefore of rapid change in fauna and flora, by the passing out of old types and the differentiation of new types. Unfortunately, in the earth's history as in human history, it is exactly these critical periods—these periods of change and revolution—the record of which is apt to be lost. In both histories, too, this is truer the farther back we go. Of the long inter- val between the Archaean and Palaeozoic, not a leaf of record has yet been recovered with any certainty; but of the interval now under dis- cussion many leaves of record have been recovered. These have been bound together in a separate volume or chapter and called the Permian. I shall regard the Permian, therefore, as essentially a transition period; its rocks were deposited during the period of commotion; its fossil types are in a state of change, though more nearly allied to the Palae- ozoic. From what has just been said, it will be anticipated that the uncon- formity of the Mesozoic on the Palaeozoic sometimes takes place be- tween the lowest Mesozoic and the Permian, and sometimes between the Permian and the Coal. The Permian, therefore
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