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frozen for want of cold, we ought at the end of each summer to have a v:hole years 
fissures in the ice. These fissures, which the ensuing winter is, according to the theory, 
to fill with blue ice, must, in summer, be filled with blue wccter. Why then are they not 
seen in summer \ The fissures are such as can produce plates of ice varying “ from a small 
fraction of an inch to several inches in thickness,” which, according to our own obseiwa- 
tions, produce lenticular masses of ice 2 feet long and 2 inches thick, or even (for 
we have seen pieces of this description) 10 feet long and 10 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 necessar}^ to the produc- 
tion of the blue veins appears to us improbable. 
§ 5. On the Belation of Slaty Cleavage to the Veined Structure. 
Within the last few years a mechanical theory of the cleavage of slate rocks has been 
gradually gaining grormd among those who have refiected 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 Geolog)-, and other 
compressed fossils in the same collection, illustrate in a remarkable manner his import- 
ant observations. The subsequent microscopic observations of Mr. Sorry, 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 de- 
veloped. More recently. Professor Haughton has endeavom-ed to give numerical 
accuracy to this theory, by computing, from the amount of the distortion of fossils, 
the magnitude 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 natmully arises, “ Is the pres- 
sure sufficient to produce the cleavage I” Sharpe appears to have despaired of an 
experimental answer to this question. “ If,” says he, “ to this conclusion it should be 
objected, that no similar results can be produced by experiment, I reply, that we have 
never tried the experiment Avith 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 Forres : — “ The experiment,” he says, “ is one Avhich the boldest pliilosopher 
would be puzzled to repeat in his laboratory ; it probably requires acres for its scope, 
and years for its accomplishment.” 
While one of us was engaged in 1855 in examining the infiuence of pressui’e upon 
