METHODS OF PALAEONTOLOGICAL INQUIRY. 6j 



more important for our purposes is the determination of the 

 climatic features, especially of the temperature and moisture. 

 The most trustworthy instruments for this determination are 

 the fossil plants, the evidence of which, though it must not be 

 uncritically accepted, is yet very valuable. Thus the Eocene 

 and early Oligocene vegetation of the interior of our continent 

 points to the prevalence of warm climates far to the north, 

 huge palms and other subtropical plants abounding in Idaho 

 and Wyoming. By White River times a change had come, not 

 extreme at all and probably slight, but yet very significant, 

 especially in view of what was to come later. The palms have 

 nearly or quite disappeared from the northern interior, a hardier 

 vegetation taking their place; and the withdrawal of the great 

 crocodiles, which had so abounded in the Eocene lakes, con- 

 firms the inference as to climatic change. 



We may often learn something of the environment from the 

 facts of geological structure, as an example will show. On the 

 summit of the divide between the White and Cheyenne Rivers in 

 South Dakota is a patch of conglomerates and hard, coarse sand- 

 stones, which have weathered into overhanging ledges, fantastic 

 amphitheaters, and cirques. These sandstones represent a sys- 

 tem of stream-channels, cut through the lake-bed. Alternations 

 in the water stages are indicated by the clay beds, which dovetail 

 in along the edges of the sandstones and were obviously formed 

 at the same time as the latter. Both sandstones and clays are 

 crowded with fossils, and both, as we have seen, were contem- 

 poraneous, and yet it is quite remarkable how different the 

 animals are; species which are common in the clays are rare 

 or absent in the sandstones, and vice versa. The explanation 

 of this curious difference is probably to be found in the infer- 

 ence that the sandstones contain principally the remains of the 

 upland fauna, which were swept down by the flooded streams 

 and entombed in the lake, while the fossils of the clays repre- 

 sent chiefly aquatic forms and species which haunted the low- 

 lying, swampy shores. That a certain amount of mingling of 

 the species should occur was inevitable, in view of the contem- 

 poraneity of the containing strata, and certain species also 

 doubtless ranged over the whole area, hill and plain and 



