Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/435

 Raasay. On the southern declivity, which shelves away from the base of the Scur, the interbedded dolerites are traversed with an irregular band of intrusive rock, which weathers into a succession of rounded knolls along the slope above the ruined hamlet of Lower Grulinn. This rock varies considerably in different places. For the most part it has a grey porphyritic base, resembling that of the grey porphyry of the Scur ; in some places, however, it becomes darker and heavier, and assumes more the character of a doleritic rock. Possibly more than one variety of rock may here have been erupted along the same line. The third intrusive mass of porphyry is shown on the map a little to the east of Laig farm. It is a compact, yellow, quartziferous rock, resembling some parts of the first- named mass, and weathering with a platy texture. Its exact relations cannot be here made out ; but it cuts through the basalt-rocks, and is thus later than they are.

Although the full importance of the intrusive bosses of felstone and quartziferous porphyry in the Tertiary volcanic series cannot be properly understood from the structure of Eigg, yet the examples which occur there are of interest, inasmuch as they are found associated with and penetrating the basalt-rocks, and thus serve to indicate the true relations of other masses which have invaded the Liassic and Oolitic strata of the Inner Hebrides at a distance from the main mass of the basalt-plateau.

β. Sheets.

Geologists are familiar with the often-quoted illustrations given by Macculloch of the way in which the trap -rocks of Skye have been thrust between the planes of the secondary strata, so as to run for a long way strictly parallel to them, appearing as regularly interstratified beds, and then to break across the strata, thereby revealing their true intrusive character*. I have already remarked that these features, which are characteristic of a certain horizon in the volcanic series, have been very commonly transferred to the whole of that series, which is cited in consequence as a kind of classical example of the intrusive nature of trap-rocks. In reality, however, the intrusive sheets are almost wholly confined to the lower portion of the igneous series, and they are quite subordinate in number and extent to the great interbedded sheets of the plateau.

So far as I have yet been able to ascertain, it is only the basalt- rocks which are ever found counterfeiting the parallelism of the true flows. The petrographical character of these rocks does not, then, differ essentially from that which they manifest when they occur as interbedded sheets. Yet, as a rule, they are more compact and closer-grained, never slaggy, and seldom amygdaloidal. Although the rock is finely crystalline thoughout, the upper and lower edges of each sheet are more close-grained than the central

illustrations are given from the east coast of Trotternish.
 * See in particular plate xvii. of his ' Western Islands,' where numerous