418 ON THE TERTIARY AND OLDER PERIDOTITES OF SCOTLAND. 



He thought the mystery about magnesian rocks was artificial. He 

 suggested that the presence of diallage in deep-seated rocks might 

 be due to hydration owing to depth, because the water could not 

 escape. Amphibolization is well illustrated in the gabbro of Corn- 

 wall, when diallage is converted into hornblende at the surface. 



The Author, in reply, said he did not mean to assert that 

 mechanical strain produced twinning, but only develops it where 

 the tendency already exists. It has been shown that this structure 

 is induced by pressure in calcite, and, if so, the same change is 

 possible in the plagioclase felspars. The case is similar with leucite 

 and boracite, in which, however, the twinning disappears on the 

 mineral returning to the original temperature of its formation. The 

 result of twinning in orthoclase is seen in microcline, and Lehmann 

 refers microcline-structure in Saxony to pressure. Hypersthene was 

 always, until lately, studied only in altered forms. Schillerized forms 

 are produced by deep-seated hydration, weathered forms by hydra- 

 tion near the surface. In reference to the alleged association of 

 diallage and magnesian minerals, he stated that several varieties of 

 augite pass into diallage. In answer to Prof. Hughes he said 

 that the veins in the specimens referred to are irregular injection- 

 veins. 



