HUGHES : ADAM SEDGWICK. 
263 
of enormous piles of rocky sediment upon compressible material in ad- 
joining- areas or to the secular shrinking of the earth’s crust) they yield 
more or less, and so every mass or particle which has or could 
assume a flattened form is forced to arrange itself in planes at right 
angles to the pressure and we get the phoenomenon of cleavage. 
When shrinkage sets in or strains, causing often cracks, another set 
of divisional plains is formed. Others besides Sedgwick, viz. : — 
MacCulloch, and Bakewell, and Phillips, and especially Jonathan 
Otley, had observed and speculated upon these cleavage, joint, and 
bedding planes, but it remained for Sedgwick to work the question 
up, and describe their manner of occurrence and relations to one 
another. The results of his investigation he published in this masterly 
work on the structure of large mineral masses. I will not attempt to 
give a resume of all his work, a mere enumeration of the seventy or 
eighty papers written by him would take more time than I could 
ask you to accord me. I ask you to bear in mind that most of what 
I have described was done about half a century ago, and to remem- 
ber what was the state of geological enquiry then. His chief work, 
however, was in the old G-rauwacke, and I think I would not be 
doing justice to my hero, or my duty to the society, which has done 
me the honour to allow me to give this short notice of his work, if 
I did not, in pointing out the progress of his enquiries into the Cam- 
brian and Silurian rocks state briefly but clearly the circumstances 
which led to the great controversy between him and Murchison — -a 
controversy which from the very personal character which it in this 
case, almost necessarily assumed, gave a tone to much of his later 
work and showed through all his later life. 
In 1835 he read conjointly with Murchison before the British 
Association a paper on “ The Silurian and Cambrian Systems, exhibit- 
ing the order in which the sedimentary strata succeed each other in 
England and Wales.” In this paper Murchison gave Ludlow, 
Wenlock, Carador, Llandeilo. Sedgwick described Upper, Middle, 
and Lower Cambrian, and further began already to speculate on the 
correlation of the North Wales series with the Cumbrian of the 
Lake District, stating that there was not sufficient evidence to allow 
him to say that the lowest rocks of the lake district were on the 
