THE GEOLOGIC DAY 427 



line was merely the straightest line he could draw, and who accord- 

 ingly thought he had squared the circle, because he found a con- 

 struction which was correct so far as he could see. It does not fol- 

 low, however, that we should give a new name every time we find 

 evidence of real difference in geologic time in the beginning of one 

 of our geologic divisions. It should be carefully noted, and left to 

 be weighed and compared with other evidences, until we are ulti- 

 mately able to place each division at each place accurately on a 

 true scale of time, which shall be to the common scale as is astrono- 

 mer's time to the local times and seasons of everyday use. 



Of these various evidences, fossils are by no means the best evi- 

 dence of strict contemporaneity. Other evidences, as good or better, 

 are: 



1. A shower of volcanic ash like that which has recently come 

 from Vesuvius, if it can be identified, is one of the best evidences 

 of contemporaneity. The same remark applies to a surface lava 

 flow. Individual lava flows of peculiar character, like the foot of 

 the Kearsarge lode, have been traced many miles in the Keweenawan, 

 and great floods out West may, I presume, be equally contempo- 

 raneous. In time to come, geologists of the future may use volcanic 

 ash-beds, among the series of shales and muds, which are overlooked 

 now, as horizon-markers of the first importance. 



2. The whole nexus of mud-flows, lavas, ash-beds, and the 

 like, which make up an eruptive epoch, would not make so exact 

 an index of contemporaneity, but in many cases are of considerable 

 value. The Keweenawan, and Triassic of the Atlantic coasts are 

 illustrations both of the value and of the danger of such correlations. 

 While indicating real contemporaneity, if the correlation is correctly 

 made, there is likely to be an eruption of indistinguishable rocks (or 

 nearly so) at widely different times. 



The error is analogous to that which may be made by confusing 

 colonies among fossils; though, on the whole, with a large enough 

 fauna there is little danger of faunal confusion. 



3. A change of climate may extend with great rapidity over a 

 province or over a very large part of the world. The glacial period 

 in Europe and America is one glaring illustration. Another is the 

 change in the Carboniferous from the hot, dry climate of the Mis- 



