1871.] GEIKIE — TEETTAKY VOLCANIC ROCKS. 295 



Eaasay. 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 com- 

 pact, yellow, quartziferous rock, resembling some parts of the first- 

 named mass, and weathering with a platy texture. Its exact rela- 

 tions 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 Tertiaiy volcanic series cannot 

 be properly understood from the structure of Eigg, yet the exam- 

 ples 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. 



/3. Sheets. 



Geologists are famiHar with the often-quoted illustrations given 

 by MaccuUoch 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 in- 

 terstratified beds, and then to break across the strata, thereby re- 

 vealing 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. Al- 

 though the rock is finely crystaUine thoughout, the upper and 

 lower edges of each sheet are more close-grained than the central 



* See in particular plate xvii. of his ' Western Islands,' where numerous 

 illustrations are given from the east coast of Trotternish. 



