Above the conglomeratic band lies a sheet of intrusive rock, which in one place has apparently cut it out, so as to rest directly upon the Torridon Sandstone (a, [Fig. 324]). The decay of the softer detrital rock underneath has caused the sill to break off in slices, which have left behind them a bold mural escarpment (b b).
The rock of this sill is a rather coarsely crystalline porphyritic olivine-dolerite, which towards the north attains a thickness of about 70 feet. It exhibits the usual prismatic jointing, but less perfectly than some of the Trotternish sills already referred to. Besides these vertical joints, it is also traversed by a system of horizontal divisional planes which, though somewhat irregular in their course, run, in a general sense, parallel to the upper and under surfaces of the sill.
It seems to have been along this transverse series of joints that a second sill (c), five or six feet thick, has been injected. The material of this younger intrusion is a black, finely crystalline dolerite or basalt, with rudely prismatic jointing. Its most striking feature, besides its regularity of position and persistency for several hundred yards as a platform along the shore, is the basalt-glass which marks both its under and upper surfaces of contact, and which is here developed upon a scale to which I have not met with an equal among the Tertiary sills of this country.
The selvage of glass appears as a black tar-like layer, varying from a mere film to two or three inches in thickness. It is found not only on the upper and under surfaces, but descends along abrupt step-like interruptions of the upper surface, a foot or more in height, as if the sill had been broken by a series of subsidences. The apparent fracture, however, is probably due to the irregularities of the passage forced for itself by the molten rock as it passed from one line of horizontal joint to another through the heart of the older sheet.
The exposed surface of black glass on the top of the younger sill exhibits long parallel lines, probably marking flow-structure, which are made conspicuous by a pale yellow ferruginous weathered crust. Portions of the larger intrusive sheet have been broken off and involved in the later rock. The younger sill disappears to the north, and is not found in the cliff of Rudha Chàrn nan Cearc, where the thick sill, lying once more on the band of conglomerate, forms a fine escarpment above the shore. Dykes of fine-grained basalt (d d) with compact chilled margins rise through both sills, together with veins which pursue a wavy upward path like strips of black ribbon.
This example, and that of the Shiant Isles already described, cannot but impress the observer with the prodigious force with which the material of the sills was injected. In these instances solid sheets of intrusive rock have subsequently been rent open, doubtless under a superincumbent pressure of many hundreds of feet of the terrestrial crust, and a new injection of molten magma has made its way into the rents thus caused. In each case, the position of the rents was obviously determined by structural lines in the older sills, but we are lost in astonishment at the energy required to split open, even along these lines, such solid crystalline masses as the thick sills, and to overcome the superincumbent pressure of so deep a pile of rock.
The isolation of a relic of the Tertiary sills on the west side of the promontory of Sleat presents some interesting problems to the mind of the geologist. The locality lies about midway between the basalt-plateau of Strathaird and that of Eigg, and some eight or nine miles in a direct line from either. The basalts cannot be proved to have once stretched continuously between Eigg and Strathaird, and to have covered this part of Sleat; but the position of the Sleat sills makes it probable that this continuation did formerly exist. The denudation of the West of Scotland since early Tertiary time has been so stupendous that I am prepared for almost any seemingly incredible evidence of its effects. There can hardly be any doubt, however, that the sills here described belong to the great platform of intrusive sheets, and that they were injected under a pile of Secondary strata, if not also of Tertiary basalts, which has here been entirely removed.
Reference may be made, in conclusion, to a not infrequent feature of the Skye sills. Like the dykes, they are often double or multiple, molten material having been successively injected along the same plane. The example just cited from the west side of Sleat illustrates one type of such compound sills. More frequently, however, the subsequent injections have been made along the floor or roof of the first sheet. Mr. Harker has found numerous cases of this structure in the Strath district. They are recognizable even from a distance by their terraced contours when seen in profile. They often vary considerably in thickness owing to the dying out or coming-in of their separate bands; while, on the other hand, single sills tend to maintain a uniform thickness for long distances, or taper away gradually. The compound arrangement of the basic sills is well brought out where acid material has been injected between the sheets, as will be more fully described in Chapter xlviii.
iii. EIGG, ARDNAMURCHAN
The phenomena of the coasts of Skye are repeated on the east side of Raasay, in Eigg, and still more magnificently along the south coast of Mull. A single example is here given ([Fig. 325]) from the east side of Eigg. Over the Jurassic sandstones (a a) a sill of basalt (1) four to six feet thick has been injected between the stratification, and another (2) two to four feet thick has forced its way across the middle of one of the bedded basalts (b b) in which it bifurcates, and above which comes the thick series of lavas of the plateau (c, d). In one of the streamlets, which exposes a section of the Jurassic strata below the volcanic escarpment, more than twenty intrusive sheets may be counted among the shales and limestones. They are sometimes not six inches thick, and seldom exceed six or eight feet.