MODIFICATION OF THEORY EXAMINED. 133 



of the two. At both periods, the greater part of 

 North-western Europe was buried under ice. We 

 know that during the last great ice-period, which was 

 undoubtedly the period of the Hessle boulder-clay, 

 the ice -sheet reached in North Germany as far as 

 Berlin; while during the period of the Purple boulder- 

 clay it advanced to about Saxony. 



The accompanying chart, reproduced from ' Climate 

 and Time,' which was sketched out during the summer 

 of 1870, shows pretty correctly the condition of North- 

 western Europe both before and after the intergiacial 

 period referred to by Mr. Wood. The observations of 

 Professor J. Geikie and Mr. A. Helland have since 

 shown, however, that the Scandinavian land-ice did 

 not pass over the Faroe Islands, as represented in the 

 chart ; but the chart has, in almost every other parti- 

 cular, been now proved by geologists to be accurate. 

 The chart exhibits in a striking manner the enormous 

 amount of ice which must have been melted off the 

 ground before the warmth of the intergiacial period 

 could even have commenced. 



The observations of Prof. Torrell, Dr. A. Penck, 

 Prof. Credner, Prof. Berendt, Dr. Jentzsch, A. Helland, 

 F. Wahnschaffe, H. Habenicht, and other geologists, 

 have shown that there are in North Germany three 

 distinct boulder-clays — an Upper, Middle, and Lower, 

 with two series of intergiacial beds. In these inter- 

 giacial beds have been found organic remains which 

 evidently indicate a mild and genial condition of 

 climate. The younger intergiacial period (the one 

 prior to the last great extension of the ice) in all 

 probability corresponds to the last intergiacial period 

 of Scotland, England, and Ireland. Intergiacial beds 

 belonging to the same period have been found in 

 Switzerland, Italy, Denmark, North America, and other 



