372 Notices of Memoirs — John Ball — Motion of Glaciers. 



II. — On the Cause of the Motion of Glaciers. 

 By John Ball, F.E.S. 



[Philosophical Magazine, February, 1871.] 



IN tlie Geological Magazine for December last we gave a short 

 notice of a paper by Mr. CroU, in which, he endeavoured to 

 prove the motion of a glacier to be molecular. To this Mr. Ball 

 replies in the paper now before us. He questions whether the 

 ordinary theory, which affirms that a glacier descends by its weight 

 through the processes of fracture and regelation, has been overthrown 

 by the arguments and observations opposed to it by Canon Moseley, 

 and which, according to Mr. CroU, successfully show the insufficiency 

 of the theory. These opinions are combated by Mr. Ball, who 

 then proceeds to explain the cause of glacier-motion as it appears to 

 him to be most consistent with the facts. He remarks that glacier- 

 ice is a substance which at the temperature of freezing is capable 

 of yielding, very slowly, to moderate pressure, and that a portion of 

 the motion of all glaciers is due to this cause, and is effected in- 

 dependently of fracture and regelation. Glacier-ice, though im- 

 perfectly solidified, is yet rigid enough to transmit very considerable 

 pressure ; but there is a limit at which pressure upon the ice (which 

 lias a fixed internal temperature of 32° Fahr.) has the effect of 

 reducing it to the liquid state. At any given moment of the progress 

 of a great glacier, especially in summer, certain points are subjected 

 to enormous pressure. The effect may either be that fracture ensues 

 at that point, and so continues further ; or else the pressure liquefies 

 a portion of the ice : and the water, even if it cannot escape, occupies 

 less space than it did before ; so that the effect of transferring 

 the maximum pressure from one point to another is accomplished. 

 It is this process which Mr. Ball has compared to the progress 

 of a huge snake, whose movements are effected not by simultaneous 

 effort at every point, but by the transmission of muscular energy 

 from one point to another. 



Ill, — The Spieorbis Limestone in the Forest of Wtre 



Coal Field. 



By Daniel Jones, F.G.S. 



[K paper read before the Manchester Geological Society, 20th December, 1870.] 



ME. Jones brings forward some additional information respecting 

 the occurrence of Spirorbis Limestone in the Forest of Wyre 

 Coal-field, occupying a district lying between the Abberley Hills 

 and Bridgenorth. This limestone, which occurs in the Upper Coal- 

 measures, has been described by Sir E. Murchison in the Shrewsbury 

 Coal-field as being about seven feet thick, and divided into two beds, 

 the uppermost of which is a compact cream-coloured rock, slightly 

 argillaceous, with a splintery conchoidal fracture and dull lustre ; 

 the lower is a cellular limestone, the cavities being filled with calc- 

 sjjar and black bitumen. Singularly enough these two beds are still 

 developed in the Forest of Wyre, where the formation is of much 



