lii PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [May IQ09,. 



consequence of mechanical pressure. Sorby made several deduc- 

 tions from this conclusion, and then proceeded to verify them: on 

 the one hand by observations on the minute structure of slate as 

 revealed in thin slices, and on the other by experiment, as when he 

 showed that certain substances, such as pipeclay mixed with scales- 

 of iron-oxide, actually acquire the property of cleavage on being 

 subjected to pressure, and further, that a definite relation exists 

 between the direction in which the pressure acts and that in which 

 the cleavage-planes are developed. They are perpendicular in the 

 experiment, as they are required to be by theory founded on obser- 

 vations in the field. Sorby's account of his work was read before 

 our Society ; and it will be a source of gratification to those whose 

 papers may not always have received the sympathy which they may 

 have thought they had a right to expect, to know that it proved too 

 novel for some of the mathematical dogmatists of the time, and 

 was printed in consequence in the Edinburgh New Philosophical 

 Journal. Sorby returned to the subject on more than one occasion^ 

 producing much additional evidence, the best known being the 

 contorted band of quartzite in the slates of Ilfracombe, which Lyell 

 afterwards figured in his ' Elements.' 



From cleavage it was a natural transition to foliation, and as a 

 result of much laborious work, Sorby was able to show that both 

 parties in a controversy on this subject were in the right, for in 

 some cases the foliation was found to follow the bedding and in 

 others the cleavage-planes. 



A much vaster realm of the obscure now began to open before 

 our explorer, and it was not long before he had penetrated into the 

 deeper mysteries of the igneous rocks. His first great paper on 

 this subject — ' Some Peculiarities in the Inner Structure of Crystals, 

 applied to the Determination of the Aqueous & Igneous origin of 

 Minerals & Rocks' — was read before the Society on December 16th, 

 1857- The most flattering testimony to its merits was contributed 

 by the chairman, who remarked 'that although he had been a 

 member of the Society since its foundation, he did not remember, 

 during the whole of that time, any communication which drew 

 so largely on their credulity.' The genial John Phillips, on the 

 other hand, who was present on that occasion and had followed 

 Sorby's earlier work on cleavage with keen appreciation, was among 

 the first to recognize the value and promise of this pregnant 

 investigation. 



It is interesting in this connexion to recall how one of our most 



