94 A NEW BRITISH HOCK. SY T. BAERON 



cut into ridges lying east and west ; it has also been breached 

 by the Tweed in a southerly direction. In this way the Eildon 

 Hills have been separated from the rest of the mass. The 

 eastern side of the Tweed valley is formed at this point by the 

 escarpment of the Old Red Sandstone, capped by igneous 

 rock; and it is part of this escarpment which forms the base 

 of the Black Hill. The opposite side of 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 Red 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. Under- 

 neath this layer, the sandstone is soft and friable for several 

 feet, but gradually passes into hard sandstone. 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 

 hill (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 conchoidal 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 abruptly 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 



