Mr. Hopkins on the Mechanism of Glacial Motion. 163 



instead of transverse, as represented in the annexed diagram. 

 This is the law recognised by Prof. Forbes in cases such as that 



Fig. 7. 



now considered. The form of the curves of fissure deduced 

 in the first case agrees with that recognised by all observers. 



22. If the mass be so constituted as to present internal 

 planes or surfaces of less cohesive power , the only case we need 

 consider is that in which these planes or surfaces are very near 

 and parallel to each other. 8uch a system may possibly be 

 formed by the alternate bands of blue and white ice described 

 by Prof. Forbes, and appears to be the only one of which there 

 is any indication. If the direction of any surface of such a 

 system, through any proposed point, approximate sufficiently 

 to perpendicularity to the line of maximum tension for that 

 point, it is manifest that the fissure will be formed along the 

 surface of less cohesion ; but if, on the contrary, this surface 

 approximate sufficiently to the direction of maximum tension, 

 it is easily seen that it will have no influence whatever on the 

 direction of fracture. Now, according to the observations of 

 Prof. Forbes, the blue bands are very approximately perpen- 

 dicular to the transverse fissures, and therefore nearly coin- 

 cident with the lines of maximum tension ; consequently 

 those bands, allowing that they form surfaces of less cohesion, 

 can exert no material influence on the formation of the trans- 

 verse fissures. 



23. From this investigation we may also see distinctly by 

 what action the lateral parts of a glacier are made to expand 

 with an expanding valley ; for the directions of greatest pi'ess- 

 ure being perpendicular to those of greatest tension, they must 

 be parallel to the fissures represented in fig. 7, and therefore 

 this greatest pressure must necessarily force out the flanks of 

 the glacier and cause the lateral expansion here spoken of. 



24. Internal state of tension or jnessure of the Mass after 

 the formation of transverse fissures. — Since the fissures will be 

 perpendicular to the direction of the maximum tension (R), 

 and therefore parallel to that of (r), it is manifest that (R) will 



