ON THE STRUCTURE AND MOTION OF GLACIERS 13 
tion of an inch to several inches in thickness,” which, according to 
our own observations, produce lenticular masses of ice 2 feet long 
and 2 inches thick, or even (for we have seen pieces of this descrip- 
tion) 10 feet long and Io inches thick; and M. Desor informs us in 
the memoir from which we have already quoted, that under the 
medial moraine of the Aar glacier, there are bands 10 inches and 
even a foot in thickness. Such fissures could not escape observation 
if they existed, but they never have been observed, and hence the 
theory which makes their pre-existence necessary to the production 
of the blue veins appears to us improbable. 
§ 5. On the Relation of Slaty Cleavage to the Vetned Structure. 
Within the last few years a mechanical theory of the cleavage of 
slate rocks has been gradually gaining ground among those who have 
reflected upon the subject. The observations of the late Daniel 
Sharpe appear to have originated this theory. He found that fossils 
contained in slate rocks were distorted in a manner which proved that 
they had suffered compression in a direction at right angles to the 
planes of cleavage. His specimens of shells, which are preserved in 
the Museum of Practical Geology, and other compressed fossils in 
the same collection, illustrate in a remarkable manner his important 
observations. The subsequent microscopic observations of Mr. Sorby, 
carried out with so much skill and patience, show convincingly that 
the effects of compression may be traced to the minutest constituents 
of the rocks in which cleavage is developed. More recently, Professor 
Haughton has endeavoured to give numerical accuracy to this theory, 
by computing, from the amount of the distortion of fossils, the magni- 
tude of the change which cleaved rocks have undergone. By the 
united testimony of these and other observers, whose researches have 
been carried out in different places, the association of cleavage and 
compression has been established in the most unequivocal manner ; 
and hence the question naturally arises, “Is the pressure sufficient to 
produce the cleavage?” Sharpe appears to have despaired of an 
experimental answer to this question. “If,” says he, “to this con- 
clusion it should be objected, that no similar results can be produced 
by experiment, I reply, that we have never tried the experiment with 
a power at all to be compared with that employed ; and that this 
may be one of the many cases where our attempts to imitate the 
operations of nature fail, owing to the feebleness of our means, and 
the shortness of the period during which we can employ them.” The 
same opinion appears to have been entertained by Professor Forbes : 
