THE BLACK HILL OF EARLSTON 53 



Hence its title Old Eed Sandstone is really justified by the 

 facts, which can hardly be said to be the case with the strata 

 just noticed. As the rock under consideration forms the upper- 

 most member of a great series of strata which were found in 

 North Britain during Devonian times, it is also fully entitled 

 to the further distinction of being called the Upper Old Red 

 Sandstone. The history of the formation of the Upper Old Eed 

 Sandstone calls for a few remarks here: — At the time when it 

 began North Britain existed under continental conditions — by 

 which is meant, not that any mountain, valley, or sea-bottom, 

 now in existence as such, had yet come into being, but that this 

 part of the Earth's surface was situated in such a manner in 

 relation to the geographical features of those times that the 

 lowlands received a smaller rainfall than was requisite for the 

 maintenance of vegetation. On the hills the case may well 

 have been somewhat different; but even there the evidence 

 shows that an arid climate, occasionally varied by heavy down- 

 pours of rain, prevailed for a very long time. Hence the 

 geographical conditions must have borne a more or less close 

 resemblance to those now existing in the arid regions of Central 

 Asia. The slopes of the hills were trenched by large wadies, 

 out of which occasional floods swept large quantities of angular 

 rock dehris to the lowlands beyond. Here and there on the 

 lowlands a few small streams managed to hold out as pools 

 from the time of one flood to that of the next. The few 

 perennial inhabitants of these pools were mostly fishes. So far 

 as is known at present these consisted of some remarkable 

 armour-plated fishes of low zoological grade, represented in the 

 area now occupied by Tweed side, by Bothriolepis obesa. This 

 animal is classed as one of the Ostracodermi, which is an 

 entirely extinct group distantly allied to the Elasmobranchs. 

 With this there certainly occurred the remarkable fish Holop- 

 tychius, another extinct genus of fishes, allied to the modern 

 Polypterus of the mid-African rivers. Probably other forms of 

 fishes may have tenanted the rivers or the pools of the part 

 now represented by Tweedside, as the remains of a long list of 

 fishes have been found elsewhere. But only these mentioned 

 have, however, yet occurred on the area under notice. 



Wind played an important part in the formation and dis- 

 tribution of the materials out of which the Upper Old Eed 



