694 JAMES P ERR IN SMITH 



Goniatites striatus of the Mountain Limestone. The genera Gas- 

 trioceras and Paralegoceras appeared in America in the zone of 

 Goniatites striatus, while they are not known in Europe before the 

 Coal Measures. But the accompanying faunas in these regions 

 are, in the main, correlative, and so the heterochronous appear- 

 ance can be detected. 



In the Upper Trias, Karnic stage, of California Halorites 

 occurs, although in both the Alps and the Himalayas it is char- 

 acteristic of the higher Noric stage. Also in California Trachy- 

 ceras and Protrachyceras occur in the zone of Tropites subbullatus , 

 mingled in the same , hand specimen with typical species of the 

 Subbullatus fauna ; in the Alps and in the Himalayas Trachyceras 

 and Protrachyceras are older than the Siibbiillatus zone, and are 

 never found in it. 



Now, when we have fossil species or short lived genera com- 

 mon to two regions, are the strata of these regions to be con- 

 sidered as synchronous? Huxley 1 advanced the theory that 

 migration, from one region to another would consume so much 

 time that a fauna might become extinct in one region before 

 it reached the other, and that since we determine the age by 

 these faunas, the time of deposition of strata assigned to the 

 same geologic age might be very different. Thus a Silurian 

 fauna might survive in one region, while a Devonian fauna flour- 

 ished in a second, and a Carboniferous fauna might be beginning 

 in a third region. But the fossiliferous beds are Silurian, or 

 Devonian, or Carboniferous in the faunal sense. This relation 

 Huxley called homotaxy, and most geologists have accepted 

 without question the validity of the hypothesis. 



Viewed in the light of modern distribution of fauna, there 

 must be something in it. The present Australian fauna is often 

 cited as an example of unreliability of the time scale when 

 based on faunas, as a survival of Quaternary life at the present 

 time ; it certainly is peculiar, for the continent has been totally 

 cut off from other regions since early Mesozoic time. This fauna, 

 however, has not dropped behind; it has gone on specializing in 



Presidential address. Quart. Jour. Geol. Soc. London, 1862. Vol. XVIII. 



