80 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



flows are not more than 6 or 8 feet ; others are 30 or 40 feet. In the vast walls that 

 form the seaward margin of the tableland of Macleod's Tables (fig. 21), fourteen successive 

 beds of basalt can be counted in a vertical section of 400 feet, which is equal to an 

 average thickuess of about 28 feet. But some of the basalts are only about 6 feet thick, 

 while others are 50 or 60. 



Each bed appears, on a cursory inspection, to retain its average thickness, and to be 

 continuous for a long distance. But I believe that this persistence is in great measure 

 deceptive. It is not often that we can follow the same bed with absolutely unbroken 

 continuity for more than a mile or two. Even in the most favourable conditions, such 

 as are afforded by a bare sea-cliff on which every bed can be seen, there occur small 

 faults, gullies where the rocks are for the time concealed, slopes of debris, and other 

 failures of continuity ; while the rocks are generally so like each other, that on the further 

 side of any such interruption, it is not always possible to make sure that we are still 

 tracing the same bed of basalt which we may have been previously following. On the 

 other hand, a careful examination of one of these great natural sections will usually 

 supply us with proofs that, while the bedded character may continue well marked, the 



< I ; ■' ' ' ; ' 5 : j i ( 



Fig. 18. — Termination of Basalt-beds, Carsaig, Mull. 



individual beds die out, and are replaced by others of similar character. On the south 

 coast of Mull, for instance, cases may be observed where the basalt of one sheet abruptly 

 wedges out, and is replaced by that of another. Where both are of the same variety of 

 rock, it requires close inspection to make out the difference between them ; but where 

 one is a green, dull, earthy, amorphous amygdaloid, and the other is a compact, black, 

 prismatic basalt, the contrast between the two beds can be recognised from a distance 

 (fig. 18). Again, along the west coast of Skye, the really lenticular character of the 

 beds can be well seen. 



In Antrim also, where similar proofs may be obtained, remarkable evidence is 

 presented of the rapid attenuation not of single beds only, but of a whole series of basalts. 

 Thus, at Ballycastle, the group of lavas known as the Lower Basalts, which underlie the 

 well-known horizon of iron-ore, are at least 350 feet thick. But, as we trace them west- 

 wards, bed after bed thins out until, a little to the west of Ballintoy, a distance of only 

 about 6 miles, the whole depth of the group has diminished to somewhere about 40 feet. 

 A decrease of more than 300 feet in six miles or 50 feet per mile points to considerable 



