Page:The Journal of geology (IA journalofgeology21894univers).pdf/396

of plants whose living representatives are known to agree closely in their period of vegetation."^[1]

Fossil plants may also, in certain cases, be used to indicate the character of the water in which the deposits were laid down. Thus, the finding of an abundance of marine diatoms in an undisturbed formation is proof that they were deposited in salt water, and the finding of diatoms only known in connection with hot springs is equal proof of former thermal activity. As an example of the last may be mentioned the finding of a large number of species of diatoms in beds of infusorial earth in Utah that are now found living in a hot spring (temperature 163° F.) in Pueblo Valley, Humboldt County, Nevada, showing that the fossil specimens must have been accumulated in a hot lake of about the same temperature.^[2]

It is quite commonly argued that during Carboniferous time there was present such a large amount of carbon-dioxide that it produced a thick veil, hiding or at least largely obscuring the direct sunlight. This extreme view is not wholly sustained by fossil plants, for the presence of strongly developed palisade parenchyma in certain leaves, as in Cordaites and many ferns, which can only be formed in direct sunlight, shows conclusively that there must have been at least gleams of sunlight penetrating the so-called veil.

Before leaving the subject it may be well to point out some of the responsibilities resting with the geologist who would avail himself of paleobotanical aid in the determination of horizons. In the first place, if it is worth while to ask an opinion of the paleobotanist, it is surely worth while for the geologist to spend time enough when making the collection he would submit, to procure at least a fair representation of the fossil flora of that horizon. To expect the paleobotanist to unravel a stratigraphic problem that has perhaps puzzled the trained stratigrapher and