The Highland Boundary Fault : Reverse or Wrench Fault?
DETAILED work which I have done in the Loch Lomond area suggests a definite answer to the above problem which is of some importance since it concerns one of the major geological features of the British Isles. The direction in which the principal stress acted in the production of the fault can be approximately determined from the pattern of structures in the Lower Old Red Sandstone conglomerates, and this supports the contention put forward in stratigraphical accounts (summarized by J. G. C. Anderson1) that the Highland Boundary Fault is a reverse fault of Caledonian age and not a Proto-Armorican wrench fault as envisaged by E. M. Anderson2.
The dynamics of faulting
It has been known for long that faults arrange themselves naturally into different classes, which have originated under different conditions of pressure in the rock mass. The object of the present paper is to show a little more clearly the connection between any system of faults and the system of forces which gave rise to it. It can be shown mathematically that any system of forces, acting within a rock which for the time being is in equilibrium, resolves itself at any particular point into three pressures or tensions (or both combined), acting across three planes which are at right angles to one another. Across these particular planes there is no tangential stress, but there will be tangential stress at that point across any other plane which may be drawn through it. There will evidently be positions of this hypothetical plane for which the tangential stress will be a maximum. It is evident that these maximum positions of the plane will have much to do with determining the directions of faults in the rock. We will therefore take the general case and investigate what the positions are. Suppose O to be any point in a rock, and let the three directions along which the pressures or tensions act (the directions perpendicular to the three planes mentioned above) be OX, OY, OZ. Let the pressures, or tensions, acting along these three directions be P, Q, R, which we will suppose positive when they denote pressures, and negative when they denote tensions.
II.—On the Geological Relations of the Secondary Strata in the Isle of Arran.
§ 1. Introduction. So much has been written on the physical structure of the Isle of Arran, that an attempt to add our own observations to those which have been already published may require some preliminary explanation*. After a careful examination, during last summer, of the north-eastern shores of the island, we thought that we discovered a sufficient proof of the existence, in that region, of a succession of formations which are the representatives of the old red sandstone, the carboniferous series, and the new red sandstone of English geologists. If this conclusion be true, the evidence upon which it is founded (depending upon the observed superposition of the several beds, and the organic remains contained in them) cannot be unimportant. For the details will not only assist in completing the natural history of Arran, but will connect its structure with that of the neighbouring main land of Scotland. We hope, also, that they will throw light upon the true relations of those masses of conglomerate which are spread out on both sides of the Murray Firth, which form a well-defined mountain chain in the southern part of Caithness, and are developed upon an enormous scale on the western coasts of Ross-shire and Sutherland. The determination of the age of these great deposits appears, after the facts published by one of the authors of this paper, to be the only thing wanted to fix the true epoch of all those interrupted fragments of secondary formations which are found in so