1858.] . NICOL EASDALE AND OBAN. Ill 



chiefly situated in the small island of Easdale ; and, the higher parts 

 of the rock being exhausted, they are now carried on at a great 

 depth below the level of the sea. 



Mineral character of the rochs. — The slates extracted for roofing- 

 purposes on Easdale and Seil are usually dark-blue or almost black, 

 with a silky lustre. They are split along tine planes of cleavage; 

 but the thin laminae are uneven and undulating. Their surfaces are 

 thus often striated or wrinkled, similarly to what is named ripple- 

 mark in other beds, but in this case clearly produced in an entirely 

 different manner. Crystals of iron-pyrites (usually cubes, but with 

 one axis often abnormally shortened) are dispersed iu more or less 

 abundance through these slates. As these ciystals are not readily 

 acted on by the atmosphere, they do not injure the durability of the 

 slates ; but, with the unevenness of the cleavage-planes, they pre- 

 vent the slates from splitting so thin, or of such large dimensions, as 

 in the Welsh quarries. 



Mixed with these fine slates are other beds, of a coarser grain, 

 almost fine greywackes, and showing so little of the fissile texture as 

 to be unfit for roofing-purposes. Other beds contain a considerable 

 amount of calcareous matter, and are known as "limestone" by 

 the workmen. In these, veins of calc-spar and quartz, sometimes 

 several inches broad, are irregularly dispersed. Similar veins of 

 quartz and calc-spar abound in the slates near Oban, intersecting 

 the strata in all directions — across, oblique, or parallel to the 

 cleavage or the bedding. Another curious set of beds, well seen in 

 Easdale, consist of a soft friable material, with a texture not firmer 

 than decayed wood, so as to be cut readily with the knife, and to 

 leave a mark on paper like black chalk. It burns white in the fire, 

 and appears to contain a considerable amount of fine carbonaceous 

 matter. Similar rocks, soiling the hands when touched, abound on 

 the Sound of Xerrera near Oban, but are firmer in texture. 



Concretions in the slate ; and Coal. — Though the crystals of pyrites 

 are generally diffused through the rock, they occasionally occur in 

 greater abundance in certain beds or portions of the strata. Some 

 beds also contain nodular masses or concretions, of various sizes, 

 imbedded in the slate. These nodules contain a considerable amount 

 of calcareous matter, and often also small veins of quartz. They are 

 harder than the slate, and are destitute of the slaty structure *. In 

 one of these beds, at a depth of 140 feet from the surface, a small 

 mass of bituminous coal has been lately found. Some fragments of 

 this coal were forwarded to me last winter by Mr. John White, the 

 intelligent overseer of the Easdale quarries. In reply to some ques- 

 tions which I put to him, he informs me that the coal, of which he 

 has a portion measuring 2i inches by 1^ and 1 inch, was surrounded 

 by a gTcasy or unctuous clay, such as is usually found in slight open- 



* Some of these calcareous nodiiles are 3 feet or more in diameter ; they he 

 in the plane of the strata, and chiefly in one bed. They affect the cleavage, 

 which curves or bends round them. Some of them contain veins or layers of 

 quartz ; and one large mass has a curious twisted appearance, like a piece of 

 soft paste squeezed through a narrow hole. — J. N., January 1859. 



