[9] Sedgwick, Cambridge Phil. Trans. ii. p. 166. Mr. Teall, Quart. Journ. Geol. Soc. xl. p. 643.
[10] Messrs. Topley and Lebour, Quart. Journ. Geol. Soc. xxxiii. p. 418.
Under the microscope the rock is seen to consist essentially of the usual minerals—plagioclase, augite and titaniferous magnetic iron-ore. An ophitic intergrowth of the augite and felspar is observable, likewise a certain quantity of micropegmatite which plays the part of groundmass between the interstices of the lath-shaped felspars. Full details of the characteristics of the component minerals and their arrangement are given by Mr. Teall in the paper already cited.
The main body of the sill is a sheet which sometimes diminishes to less than 20 feet in thickness and sometimes expands to 150 feet, but averages from 80 to 100 feet. It occasionally divides, as near Great Bavington, where it appears at the surface in two distinct beds separated by an intervening group of limestones and shales. Occasionally, as at Elf's Hill Quarry, it gives out branches which send strings into the adjacent limestone.[11]
[11] Messrs. Topley and Lebour, op. cit. [p. 413].
Although in most natural sections it seems to lie quite parallel with the strata above and below, yet a number of examples of its actual intrusion have been observed. When traced across the country, it is found not to remain on a definite horizon, but to pass transgressively across considerable thicknesses of strata. Its variations in this respect are well shown in the accompanying table of comparative sections constructed by Messrs. Topley and Lebour.[12] It will be seen that while at Harlow Hill the sill is found overlying the Great Limestone of Alston Moor, at Rugley, five miles off it lies about 1000 feet lower down, far below the position of the Tyne-bottom Limestone. Still farther north, however, the sill west of Holy Island is said to lie 800 feet above the Great Limestone and to come among the higher beds of the Carboniferous Limestone series.[13]
[12] Op. cit. plate xviii.
The Whin Sill appears generally to thicken in an easterly or north-easterly direction. There are further indications that it was intruded from east to west. Thus, at Shepherd's Gap, on the Great Roman Wall, the dolerite, coming evidently from an easterly quarter, has broken up and thrust itself beneath a bed of limestone. Again, when the sill bifurcates the branches unite towards the east or north-east.[14] The sill can be proved to thin away to the west from Teesdale to the Pennine escarpment, and in Weardale the "Little Whin Sill" diminishes from 20 feet, till in three miles it disappears.[15]