250 



THE GEOLOGIST. 



this structure had been described by Griiner in 1760, by 

 Desmarest in 1779, Scoresby in 1824, and other authors, 

 and during the last winter had been claimed as a new dis- 

 covery by Prof. Forbes, who styled it ribboned or banded 

 structure. These layers he described as always of great te- 

 nuity, forming planes more or less vertical, but always pa- 

 rallel with the length of the glacier or its retaining walls. 

 The explanation of this structure offered by Dr. Stark 

 is as follows : 



During the spring and summer months it is probable that 

 glaciers advance from 1^ to 3 feet daily, and as the valleys 

 occupied by them generally widen as they recede from the 

 higher regions, every movement would leave a space 

 between them and their containing walls ; these fissures 

 would continually fill up with fresh snow and ice, increasing 

 the breadth of the glaciers, and forming a new series of ver- 

 tical planes. The frequent occurrence of mud, gravel, and 

 fragments of rock in the same planes, was considered by 

 Dr. Stark to be much in favour of this explanation of their 

 origin. This structure, he remarked, was likely to be 

 found wherever pillars and needles of ice were met with, 

 since fissures and crevices generally divided glaciers trans- 

 versely; and in passing over rough ground, the unequal 

 pressure on a combination of transverse fissures and longi- 

 tudinal lamellse would break up the ice into vertical prisma- 

 tic columns. 



3. Horizontal combined with longitudinal and vertical strata. 

 Although no such combination as this had hitherto been 

 described, Dr. Stark thought it must exist. Horizontally 

 stratified ice was confined to elevated regions, where the 

 thickness of glaciers was three or four times greater than in 

 lower valleys. Dr. Stark inferred that these beds gradually 

 wasted away as the glacier descended until only the lower, 

 or vertically stratified, portion remained. 



