376 Professors Tyndall and Huxley on the Structure 



through the glacier ; they had all the appearance of flattened 

 cakes, and the smaller cues resembled the elongated green spots 

 exhibited by sections of ordinary roofing-slate cut perpendicular 

 to the planes of cleavage. Now it appears mechanically impos- 

 sible that a solution of continuity, such as that supposed, could 

 take the form of the detached lenticular spaces above figured. ' 



3. The fissures to which the blue veins owe their existence are 

 stated to be due to the motion of the glacier ; and as this motion 

 takes place both in summer and wuiter, it is to be inferred that 

 the fissures are produced at both seasons of the year. Now as 

 the fissures formed in winter cannot be filled with ice during 

 that season for want of luater, and as those formed in the ensuing 

 summer cannot, while summer continues, be frozen for want of 

 cold, we ought at the e;id of each summer to have a whole year's 

 fissures in the ice. These fissures, which the ensuing winter is, 

 according to the theory, to fill with blue ice, must in summer be 

 filled with blue water. Wliy then are they not seen in summer ? 

 The fissures are such as can produce plates of ice varying "from 

 a small fraction of an inch to several inches in thickness," which, 

 according to our own observations, produce lenticular masses of 

 ice 2 feet long and 2 inches thick, or even (I'or we have seen 

 pieces of this description) 10 feet long and 10 inches thick ; and 

 M. Desor informs us in the memoir from which we have already 

 quoted, that under the medial moraine of the Aar glacier, there 

 are bands 10 inches and even a foot in thickness. Such fissures 

 could not escape observation if they existed ; but they never have 

 been observed, and hence the theory which makes their pre- 

 existence necessary to the production of the blue veins appears 

 to us improbable. 



§ 5. On the Relation of .'^laty Cleavage to the I'eined Structure. 

 Within the last few years a mechanical theory of the cleavage 

 of slate rocks has been gradually gaining ground among those 

 who .have reflected upon the subject. The observations of the late 

 Daniel Sharpe appear to have originated this theory. He found 

 that fossils contained in slate rocks were distorted in a manner 

 which proved that they had sufi'ered compression in a direction at 

 right angles to the planes of cleavage. His specimens of shells, 

 which are preserved in the Museum of Practical Geology, and 

 other compressed fossils in the same collection, illustrate in a 

 remarkable manner his important observations. The subse- 

 quent microscopic observations of Mr. Sorby, can-ied out with 

 so much skill and patience, sliow convincingly that the effects of 

 compression may be traced to the minutest constituents of the 

 rocks in which cleavage is developed. More recently, Professor 

 llaughton has endeavoured to give numerical accuracy to this 



