T. Barron — A British Rock containing Nephelinc, etc. 373 



the valley is less steep, and is bounded by the Eildon Hills, which 

 are also formed of a base of Old Red Sandstone capped by igneous 

 rock. 



The junction between the Old Eed Sandstone and the igneous 

 rock is well seen in a quarry on the south side of the Black Hill. 

 It is marked by a layer of hard, white, siliceous rock twelve inches 

 thick, which, as will be shown later, has undergone some alteration 

 by contact with the lava. Underneath this layer, the sandstone is 

 soft and friable for several feet, but gradually passes into hard sand- 

 stone. In this soft rock, scales of Holoptychius are found, proving 

 the rock to be of Upper Old Red Sandstone age. 



The character of the rock in the three hills from which the 

 specimens were collected, is rather variable. The most westerly 

 bill (Middle Eildon) is composed of a reddish-purple, close-grained, 

 felsitic rock, which rises in a steep cliff towards the south and east, 

 but slopes more gradually towards the north and west. On the 

 north side, the rock shows a marked tendency to split into thin 

 plates, which give a sharp, metallic sound, when struck with the 

 hammer. So very marked is this tendency to split into thin plates, 

 that it is extremely difficult to obtain a specimen of any thickness. 

 On the south side, the rock is much more compact, and breaks with 

 a conch oidal fracture. 



The rock composing the Black Hill is somewhat different from 

 that already described. When the hill is traversed from north 

 to south, the rock is seen to vary on its two sides. On the north 

 side the hill terminates abrupthy in a vertical face of rock of a dull, 

 brownish-pink colour. The rock is traversed by well-marked joints, 

 which, in places, produce a rudely columnar structure; on weathered 

 surfaces there is a tendency to split into slabs. In the lower part 

 of the rock some exquisite flow-structure can be seen. This I was 

 able to trace for about 20 feet from the base, although at times it 

 was obscured by the lichens which cover the rock. Usually the 

 banding of the flow runs parallel with the horizontal joints in the 

 rock ; but in certain places it is much contorted. At one place it 

 deviated sharply from the horizontal line, and was traced in an 

 almost vertical direction for 20 or 30 feet, where it was lost on 

 account of the steepness of the rock. 



On the south side of the hill, the rock is of a dull, reddish-pink 

 colour, and is much more decomposed than that on the north side. 

 The felspars have, in many cases, been dissolved out, leaving their 

 casts, which are sometimes partly filled with chalcedony. This 

 character is, however, not very constant; for in certain places the 

 rock is hard and compact, ringing under the hammer and breaking 

 into thin slabs. 



Petrographical Description. 



The rocks from Middle Eildon are all of felsitic texture. On 

 a fresh fracture a few cleavage-faces of a clear felspar may be seen ; 

 and by the aid of a lens a bluish-green mineral can be recognized 

 all through the base of the rock. In some of the specimens 



