on at right 



20,000,000,000 



40,000,000,000 



6,000,000 



about 10 



•185 

 •016 

 •010 

 •001 



Revietcs — The Geological Survey. 373 



Liquid substance. Viscosity in c.g.s. units. 



Glacier ice at 0°C. . . . about 125,000,000,000,000 



CrystalliEe ice at 0° C. in a directi' 

 angles to the optic axis . 



Pitch .... 



Shoemakers' wax 



Mineral cylinder oil . 



Sperm oil at 100° F. 



Mercury at 10° C. 



Water at 20° C. 



Alcohol at 20° C. 



This table shows the wide range of viscosities to be found in 

 different substances. It will be seen that if the ice-sheets are in 

 a state of stagnation, considering the slopes available for producing 

 motion, the viscosity of the ice of which they are formed must 

 reach an enormous figure. The calculations I made to ascertain 

 the viscosity of the Swiss glaciers gave figures varying from 

 3^274 X 10'- to 292-2 X 10^-. Some of the data were certainly 

 somewhat uncertain; but it would appear that all glaciers have 

 not the same viscosity, owing, I think, to differences in the sizes 

 and arrangement of the glacier grains. We have no information, 

 as far as I am aware, of the sizes and arrangement of the glacier 

 grains of the extensive Arctic and Antarctic glaciers ; but I do not 

 think that their granular structure can be so coarse that they are in 

 any cases stagnant. 



The volume of ice discharged by a glacier of great width in 

 proportion to its thickness is, ceteris paribus, proportional to the cube 

 of its thickness. It is no doubt due to this fact that the thicknesses 

 of glaciers and ice-sheets in proportion to their areas do not vary as 

 much as one would at first sight expect them to. 



Whenever actual measurements have been made ice-masses have 

 been found to be moving under the influence of gravity. From the 

 nature of the movement which is set up the flow is clearly viscous, 

 and although calculations prove that ice in mass possesses a viscosity 

 which varies somewhat according to its structure, it is always 

 sufficiently small to allow of its more or less rapid flow. 



I^EVIE^WS- 



The Geological Survey. 

 I. — Summary of Progress of the Geological Survey of Great Britain 

 AND the Museum of Practical Geology for 1908. pp. iv, 98, 

 with 3 text-illustrations. London: printed for H.M. Stationery 

 Office, 1909. Price Is. 



COMPARED with the Summary of Progress for 1907, noticed in the 

 Geological Magazine for August last year, the present memoir 

 contains less material, to the extent indeed of 77 pages. The record 

 of the work done has been abbreviated, and this is no disadvantage, as 

 the matter in due course is for the most part reproduced in the memoirs 

 explanatory of the Geological Survey maps. 



We learn that the mapping of the Pembrokeshire portion of the 



