1858.] 



NICOL EASDALE AND OBAN. 



113 



The peculiar irregular disposition of the strata is well seen in many 

 places along the Sound of Kerrera, where the beds are rolled up, 

 as it were, into curves and broken cusps, so complex that words 

 cannot describe them, and intersected by veins of white quartz and 

 calc-spar. 



Slaty cleavage of the district. — The slaty cleavage is, in general, 

 very distinct, especially in the Easdale quarries. It seems to have 

 been mistaken for stratification byDr.Macculloch in the section of Soil, 

 given in the ' "Western Isles*.' In that work, also, we see but few 

 indications that he had any clear notion of the peculiar characters of 

 these two structures. He indeed refers to the distinction in one 

 case, illustrating it by a figure, and even mentions " the facility 

 with which the plane of the schistose division may be mistaken 

 for that of the bed ;" but it is curious to observe how seldom 

 he alludes to the cleavage, and how Kttle use he has made of the 

 " criterion" of the alternation of mineral character, '^ to ascertain 

 the position of beds of clay-slate," which he points out in the same 

 place t- Both in Easdale and Seil the cleavage dips very regularly 

 at about 60°, to E. 30° S. (true), but varies in some places by a few 

 degrees both iu amount and direction. It also changes its position 

 in passing through the calcareous bands, in which it is flatter, or 

 inclined at a lower angle. On Kerrera and near Oban, the rocks 

 also show a strong and well-marked cleavage ; but this is often con- 

 joined with other imperfect cleavages, or planes of division developed 

 in other directions. In some places near Oban three of these divi- 

 sion-planes may be seen, besides the stratification, to which they are 

 sometimes parallel, at others oblique or perpendicular. Two of the 

 best-marked of these planes meet at an angle of about 70°. . Where 

 they are fully formed, the rock breaks into very irregular fragments. 

 In this part of the formation also, we do not find that constancy in 

 the position of the cleavage which prevails at Easdale. It varies 

 very considerably, even in a limited section and in contiguous beds. 

 Thus, in one section of only a few yards in length, I noted the fol- 

 lowing dips of the cleavage in separate beds, in descending order : 

 the beds are marked by letters ; and the dips are magnetic, and not 

 corrected for variation. The dip of the beds a to h was 75° S. 

 35° E.; of the lower beds, i, Jc, 50° S. 37° E., as noted at the time. 



a . . Cleavage-dip 57° N. 



h about 65° N.? 



€ 16° S. 30° W. 



d 45°N'.10°W. 



e 15° S. 15° E. 



/ 65°N. 10°W. 



g about the same. 



h first cleavage 56° N. 15° W. 

 second ditto 30° S. 10° E. 



i 55°N.10°W. 



Jc 48°N'.16°W. 



Easdale ... 60° S. 30° E. 

 BallahuHsh . 65° W. 



This section (and it is by no means a solitary example) seems to 

 me important in several respects. It shows that the cleavage-planes, 

 even in alternating beds, may dip in opposite directions, in some 



* PL xxii. fig. 1. t Western Isles, vol. ii. pp. 242, 243, pi. xxii. fig. 6. 



VOL. XV. PART I. I 



