224 R. M. Deeley—Boulder Clay in Derby. 
At the same time and on the same assumptions, the depth of the 
surface at which stretching is greatest would be about 72 miles, this 
surface being less than a mile below the surface of greatest rate of 
cooling. 
We may sum up the results obtained for this period as follows, 
assuming the surface of the earth to be smooth and spherical: 
Folding by lateral pressure is greatest at the surface. It diminishes 
as the depth increases until, at a depth between four and five miles, 
it vanishes. Below this depth, folding by lateral pressure gives 
place to stretching by lateral tension, and the stretching increases as 
the depth increases until it is a maximum at a depth of about 72 
miles. Below this, again, the stretching begins to decrease, and it 
continues decreasmg, until it practically vanishes at a depth of about 
200 miles. 
If the contraction theory be true, the most important evidence 
which the surface of zero-strain gives us is that relating to the 
extremely superficial nature of the forces which produce our 
mountain-ranges. Some geologists have regarded the slight depth 
of the surface as a strong argument against the theory. It seems to 
me that they are somewhat hasty in coming to this conclusion, that 
they do not take sufficiently into account our utter ignorance on many 
important points. It cannot be denied that the theory is surrounded 
with great and serious difficulties, but I submit that there is a 
difference between objections which cannot be met and difficulties 
which have not yet been solved; and I cannot help thinking that 
these difficulties, if treated as subjects for future investigation, will 
sooner or later be removed. The contraction theory is attractive not 
only from its beauty and its simple grandeur. It explains so many 
phenomena in the evolution of mountain-chains ; so many apparently 
unconnected facts are grouped together by its guidance; that the 
reasons must be weighty indeed which shall lead us to reject it. 
1X.—An Exposure or Mippir anp Newer PLEIsTtocenE BovuLpER 
Cray In Dersy. 
By R. M. Dzetey, F.G.S. 
OME very interesting deposits of Pleistocene age have lately 
\-) been exposed on the Burton Road, Derby. The road rises on 
the north side of Mill Hill, and near the top, at the height of 260 
feet, cuts into a mass of Boulder-clay, which is, or was, well shown 
in the cuttings for the new roads leading into Byron Street. Another 
outlier of the same clay is exposed in Littleover Lane to the south- 
west. The main mass of the deposit cut into on the Burton Road is 
ared morainic clay with boulders; apparently a subaerial moraine 
subsequently modified by the passage over it of land ice. Unlike 
the tough, silty, red and blue aqueous Boulder Clays so plentifully 
spread over the Midland counties, it shows little or no signs of 
aqueous action. Sometimes it has a banded or streaked appearance, 
but this seems to be due rather to a crushing or pressing-out action 
than to original conditions of deposition. In this respect it much 
