IN MEMORTAM : HEXRY CLIFTON SORBY. 
249 
little has been added since, whereas the study of microscopic 
petrology has attained to transcendent importance and a literature 
of vast extent. 
It is difficult to select from Sorby's many other geological 
researches those most worthy of mention in this brief notice, but 
his discovery of the true cause of slaty cleavage is one that cannot 
be omitted. Prior to its publication various chemical, electrical, 
and other agents had been invoked, but Sorby's explanation 
that lateral pressure, by causing the rotation of inequiaxed parti- 
cles of the rock materials, had imparted a tendency to cleave 
in planes perpendicular to the pressure, agreed so well A\ath 
observation in the field and with the minute structure of slate, that 
nothing has since been seriously urged against it, though greater 
importance may now be assigned to " shear " than formerly. 
The last paper emanating from Sorby's fertile mind appeared 
in the Quarterly Journal of the Geological Society after his 
death. It deals mth the phenomena of the deposition of sands 
and clays, and is based upon observations and experiments 
extending over a period of nearly 60 years. Many of the ex- 
periments, of a delicate and refined character, were conducted 
during the last few months of his life, while the author was lying 
stricken with partial paralysis. This paper displays all those 
qualities of the ingenious experimenter and acute reasoner that 
we have been accustomed through so many years to associate 
with our ideas of Sorby and his work, and if in some instances 
the final demonstration seems not quite complete, we may well 
attribute the defect to the peculiar circumstances under which 
the paper was wTitten, and the eagerness of the author to finish 
what he declared to the present writer to be the crowTiing work 
of his life. 
Mention must be made of one other achievement of Sorby's, 
the discovery by means of pohshed and etched surfaces of metals 
of the true structure of iron and steel. The section (Plate XXVII., 
Fig. 2, and Plate XXVIIL, Fig. 2) is inscribed with a diamond 
(Sorby's general custom with his microscopic preparations) : 
" This is the first good object I made." These words are 
sufficient to invest the illustration with the utmost interest, 
not only to metallurgists, to whom his work has been 
