and the Birth of Microscojncal Petrology. 197 



reflecting on things in general the simplest possible explanation of the 

 whole flashed across my mind. I immediately went to my work-room, 

 mixed some small pieces of coloured paper with wet pipeclay, and on 

 compressing them in the manner that slate rocks are proved to have 

 been compressed, I found that I obtained a very good representation 

 of the characteristic structure on which their cleavage depends. From 

 that moment forwards the whole theory of cleavage took a new shape 

 in my mind, and after studying by experiment, with the microscope 

 and in the field, those facts which this new hypothesis indicated as 

 important, in a few years I had the satisfaction of finding that it was 

 universally accepted as a satisfactory explanation of one of the great 

 phenomena of geology." (Essay on " Unencumbered Research.") 



When Sorby brought forward his theory of slaty cleavage it was 

 met, as he informs us,^ with great opposition on the pai't of those in 

 authority, and in consequence of this opposition he withdrew the 

 paper which he had sent to the Geological Society, and forwax'ded it 

 to the Edinburgh Philosophical Journal.* 



The studj^ of the microscopical structure of slates naturally led to 

 the examination, in the same way, of the schists, and after some 

 years of work upon them Sorby published his papers on mica-schist, 

 in which he enunciated his theory of stratification- and cleavage- 

 foliation.' It is worthy of note that in these papers he shows that he 

 was already alive to the great importance of studying the nature of the 

 cavities contained in the crystals of rocks, and the substances which 

 they contain. In his paper on mica-schist he proved that some cavities 

 contain water, and in a paper published about the same time on the 

 Magnesian Limestone ^ he indicated that the cavities contain aqueous 

 solutions. It was at a later date (1869) that he proved carbon dioxide 

 in a liquid state to be present in some of these cavities. 



It will be seen from the above account that Sorby's work in 

 microscopic petrography commenced in 1849, was carried on by him 

 continuously down to 1856, and, therefore, that the statements which 

 have been made that in the year 1856 he first learned how to cut 

 sections from rocks from Alexander Brj'son, who had in turn been 

 taught by William Mcol,^ have no foundation in fact. 



Everyone will acknowledge the valuable work done by the ingenious 

 William Nicol in preparing the sections of fossil wood for Witham's 

 work on the subject. I am not aware, however, that ISTicol anywhere 

 claims to have invented the method. Alexander Bryson, indeed, in 



1 "Fifty Years of Scientific Eesearch," pp. 6-6. 



2 Ediub. New Phil. Journ., Iv (1853), pp. 137-150 ; Proc. W. Yorks. Geol. Soc, 

 iii (1853), pp. 300-311 ; Pliil. Mag., xii (1856), pp. 127-129. In 1876 I induced 

 Sorby to exhibit some of the artificial products and specimens on which he based his 

 conclusions concerning slates and schists to the Loan Exhibition of Scientific Objects, 

 and these are still to be seen in the Science division (Geological Section) of the 

 Victoria and Albert Museum, South Kensington. 



^ Brit. Assoc. Rep. (1856), pt. 2, p. 78 ; Edinb. New. Phil. Journ., 2nd ed., iv, 

 p. 339. 



1 Brit. Assoc. Rep. (1856), pt. 2, p. 77. 



= "The Founders of Geology," 1st ed. (1897), pp. 276-280, and 2nd ed. (1905), 

 pp. 462, etc. "The History of the Geological Society of London" (1907), 

 pp. 170-172. 



