Yol. 52.] BASALT-rLATEAUX OF NORTH-WESTERN EUROPE. 



377 



sometimes taken place at right angles to the surface of contact 

 with the underlying strata. But the most remarkable feature 

 in this sill is the surface which it presents to the oyster-beds on 

 which it rests. The fine-grained dark dolerite has there assumed 

 the aspect of a sheet of iron-slag, with a smooth or wrinkled, 

 twisted, ropy surface, which displays fine curving flow-lines. 

 No one looking at a detached specimen of this surface would be 

 ready to admit that it could possibly have come from anything 

 but a true lava-stream that flowed out at the surface. The 

 contours of a viscous lava are here precisely reproduced on the 

 under surface of a massive sill. 



A little farther south the promontory of Eist, which forms the 

 western breakwater of Moonen Bay, is formed by another important 

 sill or group of sills which has insinuated itself among shales, shell- 

 limestones, and shaly sandstones, fall of Ostrea hebridica, Oyrena 

 aurata, etc., and belonging to the Loch Statfin group of the Great 

 Oolite Series. The shore-cliff below the waterfall affords the 

 section given in fig. 22, illustrating the manner in which a 

 thick intrusive sheet may sometimes give off thin veins from its 

 mass. The rock attains on the Eist promontory a thickness of 



Fig. 22. — Upper part of sill in Moonen Bay, Waternish, Slcye, 

 showing the divergence of veins. 



a. False-bedded shaly sandstone ; b. Shell-limestone (Great Ooiite Series) ; 

 c. Dolerite-sill ; dd. Veins proceeding from the sill. Length of section = 

 about 5 yards. 



probably at least 100 feet, where it is thickest and undivided. But 

 the two main sheets, or branches of one great sheet, on this peninsula 

 have probably an united depth of more than 300 feet. Landwards 

 the rock splits up and encloses cakes of the Jurassic strata. It 

 possesses the usual prismatic structure and doleritic composition. 

 In Moonen Bay, as shown in fig. 22, it presonts a banded structure, 

 marked especially by alternation of lines of amygdules and layers of 

 more compact and solid dolerite, with occasional enclosed cakes of 

 baked shale or sandstone. Its upper surface is somewhat uneven, 

 and from it are given off narrow, wavy, ribbon-liko veins (d), from 



