G72 
is usually more completely decomposed, and fills up the 
spaces amongst the grains of quartz sand. It therefore 
appears as if the rock had been originally formed from a 
mixture of quartz sand and felspar sand, but, after depo- 
sition, the felspar having been decomposed into a clay-like 
material, has been forced by the pressure of the super- 
incumbent rocks into the spaces between the grains of quartz 
sand ; which change would, of course, reduce the bulk of 
the deposit. This fact affords a most satisfactory explana- 
tion of what at first appeared to be a very anomalous 
circumstance. Such sand as the finer grained beds of the 
millstone-grit are composed of, rests at an angle of about 
34 degrees when thrown down on a slope in water so as to 
form the various stratula of drift-bedding, whereas, in the 
rocks themselves, that angle is now only about 24 degrees. 
At first sight this appears to throw some doubt on the 
accuracy of the explanation of the manner of its formation 
given above ; but I have found by suitable experiments that 
when such sand was originally deposited, the spaces between 
the grains would amount to as much as 75 parts for each 100 
of solid sand ; whereas, now that the felspar sand has been 
decomposed and forced into the empty spaces, they amount 
to only 17 percent. Therefore, the thickness of the rock 
must have been diminished in the proportion of 175 to 117, 
or to about two-thirds of the original ; and thus the tangent 
of the angle at which the stratula of the drift-bedding 
are inclined to the true plane of horizontal stratification 
would be two-thirds of that of the original angle. If then, 
as already mentioned, this was 34 degrees, it would have 
been changed to about 24J degrees, which agrees so closely 
with what is actually the case, as to prove, in a most striking 
manner, the truth of the explanation I have offered. 
I have made a very extensive series of observations of 
the direction of the currents present during the deposition 
