380 THE JOURNAL OF GEOLOGY. 



of plants whose living representatives are known to agree closely 

 in their period of vegetation."^ 



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 undis- 

 turbed 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 exam- 

 ple 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, Humbolt County, Nevada, showing that the fossil 

 specimens must have been accumulated in a hot lake of about 

 the same temperature.^ 



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. 



LEGITIMATE FIELD OF PALEOBOTANY. 



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 



' A. C. Seward. Fossil Plants as Tests of Climate, p. 20. 

 ''Am. Journ. Sci., 3d sen, Vol. IV., 1872, p. 148. 



