Ix Obituary Notices of Fellows deceased. 
have been followed. While many developments and improvements of his 
methods have been introduced, he is everywhere acknowledged to have been 
the “ Father of Modern Petrography.” To have entirely revolutionised this 
important branch of research and to have opened a new and boundless field 
of investigation into the past history of our globe will ever constitute his 
chief claim to a high place among his scientific contemporaries. 
One of the geological problems which as far back as 1851 had interested Sorby 
was that of the origin of the slaty cleavage of rocks. This subject had been 
discussed by various observers, notably by Adam Sedgwick, who worked out 
with great skill the distribution of the chief lines of cleavage that have 
affected the rocks of Wales. He showed the intimate relation between the 
strike and the cleavage of large disturbed masses, and thus prepared the way 
for the true solution of the problem, although he himself favoured the 
notion that the structure was the result of the action of electric currents. 
Daniel Sharpe subsequently insisted that cleavage must be due to mechanical 
pressure, but as Sorby remarks, “little notice was taken of what he said, 
because he did not show that the ultimate structure of the rock was really 
such as would be produced by this cause.” This relation of the effect to its 
producing cause was first experimentally demonstrated by Sorby. He 
satisfied himself that cleavage has no connection with electric currents, but 
is simply due to great mechanical pressure, whereby this structure has been 
superinduced in rocks along planes perpendicular to the direction of the 
pressure. He found the microscopic structure of cleaved rocks entirely to 
support this view, which he further illustrated and confirmed by ingenious 
experiments. Thus, by mixing scales of oxide of iron with pipeclay and 
subjecting the mass to strong lateral pressure he obtained a perfect cleavage 
structure. In his paper descriptive of these observations, published in 1853, 
he dwelt on the proofs which, as thus interpreted, the cleavage structure 
furnishes of the gigantic compression undergone by mountain masses during 
their elevation. His contention was soon afterwards supported by Tyndall 
and others, who showed that even in homogeneous substances like beeswax 
a cleaved structure could be induced by mechanical pressure. Hence, for 
more than half a century cleavage has now been recognised as one of the 
most convincing proofs of the enormous compression which the more disturbed 
parts of the earth’s crust have undergone. For the establishment of this fact 
geologists are, in the first place, indebted to Sorby. 
In the course of his investigation of the minute structures of rocks, and 
with the accumulating evidence before him of the enormous pressure to 
which many parts of the earth’s crust have been subjected, it occurred to him 
that the mechanical force involved in great subterranean pressure may have 
been partly resolved into chemical action, as in other circumstances it might be 
resolved into heat, electricity, or other modification of force, and that in this 
way various puzzling appearances in rocks might receive an explanation. 
Accordingly, in his usual way, he set to work to test this hypothesis by 
direct experiment. After making a large series of investigations, he succeeded 
