66 THE JOURNAL OF GEOLOGY. 



live, have great corroborative weight, when found in association 

 with others. In all cases, much discretion much be used in the 

 interpretation of these criteria. They may be enumerated under 

 several specific heads. 



( I ) Forest Beds. Beds of vegetal deposits or old soils are 

 frequently found between layers of glacial drift. This is one of 

 the criteria most commonly cited, because it is of common occur- 

 rence and easy of recognition. The advocates of the unity of 

 the glacial period maintain that such beds of organic matter 

 might become interbedded with morainic debris during minor 

 oscillations of the ice's edge. The phenomena of existing gla- 

 ciers make it evident that forest beds or soils might be enclosed 

 by the deposits of an oscillating ice edge. By repeated oscilla- 

 tions of the ice's edge during the general retreat of the ice, such 

 vegetal beds might become interstratified with glacial drift more 

 or less frequently over all the area once covered by the ice, and 

 from which it has now disappeared. The mere presence of vege- 

 table material between beds of drift is therefore no proof of dis- 

 tinct ice epochs. This does not destroy the value of the vege- 

 tal beds as a criterion for the recognition of distinct ice epochs, 

 but it makes caution necessary in its application. It does not 

 follow that, since some inter-drift forest-beds do not prove inter- 

 glacial epochs, none do. The question is not how forest-beds 

 might originate, but how existing forest-beds did originate. 



Where the plant-remains found in the relations indicated are 

 so well preserved as to make identification of the species possi- 

 ble, we have a means of determining, with some degree of 

 accuracy, the climatic conditions which must have obtained at the 

 place where the plants grew during the time of their life. If 

 these interbedded plant-remains are of such a character as to 

 indicate a temperate climate, we can not suppose that they grew 

 at the immediate edge of the ice, and therefore that they were 

 buried beneath its oscillating margin. To be specific, if the 

 inter-drift plant remains in any given locality of the area once 

 covered by ice are such as to indicate a climate as zvarm as the 

 present in the same locality, the ice must have receded so far to 



