Prominent Yorkshire Workers — Henry Clifton Sorhy. 139 



In the following- year, 1850, the first of a lengthy series of 

 papers on the microscopic structure of rocks appeared, and 

 dealt with the Calcareous Grit at Scarborough, and even at that 

 period practically all the methods of examining rock sections 

 known to-day had been developed by Dr. Sorby. 



A year later some papers in the Quarterly Journal of the 

 'Geological Society, dealing with slaty cleavage, directed his 

 attention to that subject. Up to that time various theories had 

 been advanced to account for this structure, and at the Museum 

 of Practical Geology an experiment was made which ' proved ' 

 that cleavage was ' due to the action of weak electric currents 

 passing through deposits ! ' The late Sir Henry de la Beche 

 told Sorby that the question had been thoroughly settled at 

 the Museum. Still working on his own lines, however, Sorby 

 ■eventually demonstrated that ' slaty cleavage was due to 

 mechanical pressure, acting in a peculiar way and developing- 

 its characteristic structure in a plane perpendicular to it.' His 

 paper or the subject was sent to the Geological Society, but 

 the then President (Wm. Hopkins) had a theory of his own 

 that cleavage was developed at an angle of 45 degrees to the 

 pressure ! A leng^thy correspondence followed, and eventuall}' 

 ithe paper was withdrawn and published elsewhere. Since that 

 time Dr. Sorby has heard nothing of either Mr. Hopkins' or the 

 .electric theory of slaty cleavage ! Work amongst the schistose 

 icrystalline rocks was next taken up with good results. 



His examinations of thin sections of limestone rocks showed 

 that a knowledge of the microscopic structure of shells, corals, 

 .and other marine calcareous organisms was necessary before 

 the rock sections could be properly understood. In this connec- 

 tion it soon became evident that the question as to whether the 

 shells were of calcite or aragonite was a matter of paramount 

 importance. This subject has since been followed up by Prof. 

 P. F. Kendall.* 



The microscopic structure of minerals then occupied his 

 .attention, his work thereon being- such that on the formation of 

 the Mineralogical Society of Great Britain and Ireland he was 

 elected the first President. Sorby was the first to point out the 

 existence in certain igneous rocks of what he called ' glass 

 cavities,' analogous to ' fluid cavities,' only that when the 

 crystalline minerals were formed they caught up a liquid which 

 on cooling solidified into glass. It was thus proved that the 



* " Calcareous Org-ani.sm.s," Geol. Mag., Feb. 1888. 

 jgo6 May i. 



