liv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I909,. 



Later the same phenomenon of allotropy is turned to a more useful 

 account, as in the study of the history of various limestones, Avhich 

 culminated in the Address delivered at our Anniversary Meeting 

 in 1879, when the important part played by aragonite was 

 pointed out. 



The study of pseudomorphism was also a favourite study, and led 

 to an explanation of the Cleveland Hill ironstone as a replacement- 

 product of limestone. It is characteristic of all Sorby's work that 

 observation, whenever possible, was accompanied by experiment, 

 and in this case experiments were started which showed that, given 

 time enough — these lasted 37 years — carbonate of iron in solution 

 will replace carbonate of lime at ordinary temperatures. 



Reflection on some of the changes which have affected rock- 

 masses known to have been subject to great pressure led to an 

 enquiry into the correlation of mechanical and chemical forces, and 

 this formed the subject of the Bakerian Lecture which was delivered 

 before the Royal Society in 1863. 



The unknown liquid, distinguished by its extraordinarily high 

 coefficient of expansion, which Erewster had discovered in crystals 

 of tourmaline, was identified by Sorby with carbon-dioxide. Had 

 he only pushed his enquiries into this matter a little further, he 

 would in all probability have anticipated Andrews in some of his 

 discoveries with regard to the critical point. 



Many facts of interest were elicited by his examination of loose 

 sands ; such, for instance, as the characteristic rounding of the 

 quartz grains in deserts, and the formation of crystalline sandstones 

 by a subsequent deposit of quartz, which renewed the growth of 

 the original grains. 



The microscopic investigation of igneous rocks found its natural 

 continuation in a similar investigation of meteorities. The results, 

 which are of great interest, may be given almost in his own words : — 



'A most careful study of their microscopical structure leads me,' he wrote, 'to 

 conclude that their constituents originally existed at such a high temperature 

 that they must have been in a state of vapour. . . . On cooling, this vapour 

 condensed into a sort of cometary cloud, formed of small crystals and minute 

 drops of melted stony matter, which afterwards became more or less . . . 

 crystalline. This cloud was in a state of great commotion, and the particles 

 moving with great velocity were often broken by collision. After collecting 

 together to form large masses, heat, generated by mutual impact or . . [other- 

 wise] . . gave rise to a variable amount of metamorphism. In some few cases, 

 when the whole mass became fused, all evidence of a previous history was 

 obliterated ; and on solidification a structure was j^roduced, quite similar to 

 that of terrestrial volcanic rocks. Such metamorphosed or fused masses wer 



