220 C. Davison—Secular Straining of the Earth. 
variety of composition. Chlorite is much more abundant in some of 
them. Ilmenite is almost universal, and in many cases is as abund- 
ant and as full of sagenite as is the ottrelite-slate. Garnet is very 
abundant in some cases, and most of all in specimens taken from 
“near the contact with one or other of the igneous rocks described in 
a recent paper. 
This garnet is all colourless, or very nearly so. It is in the form of. 
very small irregular grains with only here and there a solitary case 
of any crystal-form. 
Tourmaline is much increased in amount near the contacts, and is 
in some cases the main constituent after the sericite. 
Rutile is present in large amount in all the phyllites of the dis- 
trict, as minute crystals, and, in some cases, also in large grains and 
tablets. 
In none of the many specimens examined by me, have I come 
across any ottrelite, except in the one case described. It is hardly 
likely that it is limited to the one particular quarry, but I have not 
been so fortunate as to find it anywhere else. 
There are several beds of black, shaly rock in among the sericite 
phyllites at various points. They are almost wholly siliceous in 
nature, very fine-grained quartzites cemented and veined with in- 
filtrated silica and permeated through and through by fine carbon- 
aceous matter. These rocks contain but little of any other minerals, 
and do not present any special interest. 
VIII.—On tue Securar Srraintinc oF THE Harta. I.! 
By Cuarues Davison, M.A., 
Mathematical Master at King Edward’s High School, Birmingham. 
yh early as the times of Descartes (1668) and Newton (1681), the 
“settling and shrinking of the whole globe after the upper 
regions or surface began to be hard,”* was held a sufficient cause for 
the formation of mountain-chains. Following the growth of our 
knowledge of mountain-structure, the contraction theory has been 
rediscovered several times in the present century. It has been 
worked out in great detail by Elie de Beaumont, Prevost, Delabeche 
and others; but, above all, by J. D. Dana, the real founder of the 
theory, in an admirable series of papers extending over the last 
forty-two years. 
' Read before the Birmingham Philosophical Society on Feb. 14, 1889. In this 
paper I have attempted to give an account of part of a paper ‘‘ On the Distribution 
of Strain in the Earth’s Crust resulting trom Secular Cooling, etc.,’’ read before the 
Royal Society on May 5, 1887 (Phil. Trans. 1887, A. pp. 231-242). The reasoning 
in ch. xi. of Mr. T. Mellard Reade’s work on ‘The Origin of Mountain Ranges”’ 
(1886) shows that he had previously perceived the existence of a surface of zero- 
strain in the earth’s crust, separating an outer region of crushing from an inner 
region of stretching. In his well-known paper, ‘‘On the Formation of Alpine 
Valleys and Alpine Lakes’? (Phil. Mag., Feb. 1863, 4th ser. vol. xxv. p. 97), Mr. 
John Ball arrived at the conclusion that folding by lateral pressure diminishes as the 
depth from the surface of the earth increases, until it becomes insensible. 
* Brewster’s ‘* Memoirs of Sir Isaac Newton,’’ vol. ii. Appendix 4; Nature, vol. 
XXXVill. p. 30. 
