210 Report of the State Geologist. 



that Penck has been able to trace the limit of perpetual snow in 

 the principal mountain regions, and to show that that line was 

 much lower than at the present epoch. The invasion of temper- 

 ate regions by arctic forms applies not only to the marine fauna 

 but also to the terrestrial forms, especially the flora which here 

 furnishes valuable data; the plants of the glacial epoch, as 

 JBetula nana^ Hyjpnum groenlandicum^ Hypnum sarmentosurrb^ 

 are arctic plants, successors of the Firs, Yews, etc., which con- 

 stitute a temperate flora, and which migrated temporarily 

 toward the south to return afterward into our regions. 



The hypothesis of the local formation of the arctic fauna 

 could not, in any way, explain the southward migration of the 

 preceding fauna and flora, and the return of a portion of those 

 forms at the end of the period of the great glaciers. The arctic 

 fauna must have begun its existence in the boreal regions. 



The conclusion from what precedes is evident and absolute. 

 The study of faunas and floras demonstrates that the surface of 

 the earth has been subjected to a gradual cooling process from 

 the most ancient periods. The climatic zones, at the beginnirg 

 so indistinct that their existence might be doubted and still is 

 matter for discussion, have become more and more pronounced 

 to the present day. With regard to Europe, the period imme- 

 diately preceding the one in which we live has been the only one 

 which was some degrees colder. 



This law is well known ; it has been frequently formulated. 

 But we have thought it interesting to point out that in these 

 last years the palaeontologic comparisons which had for their 

 object the inductions relating to the climatic conditions of 

 ancient epochs, are conducted with minute care by the most 

 experienced observers It is not deemed sufiicient to indicate 

 in general with what expression the phenomena have manifested 

 themselves ; the endeavor at the present day is to push pre- 

 cision as far as possible, and to form an idea of the multiplex 

 circumstances which have brought about the constitution of the 

 varied faunas and floras found in the diverse formations of .all 

 the regions of the globe. 



