AXE GEAVELS OF ABE-TUX, ETC., TS MEEL. 



281 



About 200 yards to the east the sedimentary beds are traversed by 

 the descending intrusive basalt, and are reduced to 13 feet in thick- 

 ness with no coarse gravel ; and 200 feet beyond this they barely 

 measure 1 feet 6 inches of fissile sandstone, with compact shale at the 

 base, containing well-preserved examples of Equisetum CampbeUii, 

 Forbes. A quarter of a mile further east the gravels again thicken 

 to over 20 feet, but the beds corresponding to the leaf-beds are not 

 visible, and could not be rendered so without considerable labour. 

 Their horizon soon after passes under the sea-level, and on its re- 

 appearance in the next headland, Rudha Dubh, the rudely columnar 

 basalt rests on the eroded and partly decomposed surface of the 

 amorphous basalt, without the slightest trace of gravels or other 

 aqueous deposit. Search still further up the loch has been equally 

 vain. 



Inland, however, in a direction almost due south and south-east, 

 the position of two seams of lignite is indicated on the sketch-map 

 and section accompanying the Duke of Argyll's paper, and he has 

 quite recently caused an excavation to be made near Bunessan, 

 which gave a considerable thickness, without reaching bottom, of 

 black shale and apparently decomposed basalt *. 



Following up the Traps still further in the same direction across 

 the Ross, we meet with a similar series of beds on the Carsaig coast, 

 though with some important modifications. The sedimentary series 

 is first visible at the Carsaig Arches, where it consists of from 10 to 

 12 feet of indurated river-sand f. An upthrust has displaced and 

 raised it almost to the crown of the western arch, but in the long 

 arch it occupies its true position, with its base slightly below the sea- 

 level. It rises eastward, and at the same time increases rapidly in 

 thickness, and where next visible gives the following section (fig. 7). 



In this section we have apparently the same flow of rudely colum- 

 nar basalt (d), maintaining a thickness of about 25 feet ; then 7 or 

 8 feet of indurated sandy mud (e), passing into a sand ; and then a 

 grit (/) composed entirely of angular pieces of broken-down flints, 

 with occasional boulders of rock and large flints at the base. This 

 weathers down to a slope of 25°, and rests on a compact mass 9 feet 

 6 inches thick (g), of almost unrolled flints of all sizes up to that of 

 a quartern loaf, in a matrix apparently of broken-down Trap and 

 occasional boulders of the same. Under the flints there is 2 feet of 

 impure bedded sand with indistinct vegetable markings (7i). An 

 exposure a little nearer the Arches only differs in being of a slightly 

 finer material, with a thin band of lignite at the base. 



* [This "was taken for a fire-clay when the diggings "were commenced. After 

 digesting with acid a precipitate of alumina and ferric oxide remained, amount- 

 ing to 3076 per cent. Some aluminous silicates probably remained undecom- 

 posed. Under the microscope it was seen to consist of various mineral particles, 

 some being clear and colourless felspar. The flaky green mineral present in 

 quantity, is an alteration-product,, fibres of it penetrating other minerals. — Gr. 0.] 



t Earl Compton- (Trans. Greol. Soc. vol. v. part 2, p. 373) mentions that the 

 arch is 60 feet high, 150 feet long, and from 50 to 60 feet wide, and of basalt, 

 standing on green sand. There are illustrations on the accompanying plates 20 

 and 21. Prof. Judd had, of course, recognized the fluviatile origin of the Carsaig 

 gravels (Quart. Journ. Geol. Soc. vol. xxx. p. 229). 



Q.J.G.S. No. 170. jj 



