54 THE BLACK HILL OF EARLSTON 



Sandstone was formed. Many of the sandstones are largely 

 composed of sand grains which have undergone prolonged wear 

 against each other, caused by the bowling action of the wind. 

 The beds of sandstone, too, as a whole, bear a very close 

 resemblance in the disposition of their component layers, as 

 well as in some other respects, to those of the sand dunes 

 occurring in modern deserts. 



We may, therefore, confidently regard the Upper Old Eed 

 Sandstone as being, in the main, a rock of desert origin. 

 During the occasional, or rare, heavy downpours of rain above 

 referred to, the rock material broken up by the heavy strain 

 arising from the great diurnal range of temperature, insepar- 

 able from an arid climate, was swept out on to the lowlands. 

 On similar occasions rivers extended their courses beyond their 

 usual limits, and the fishes inhabiting those rivers were, 

 consequently, transported into areas of tho lowlands, where 

 their waters were quickly dried up. The action of the wind 

 kept the sand always on the move, and, at the same time, 

 continually furnished new supplies, which were derived mostly 

 from the waste of rocks of the adjoining uplands. 



So, in time, the lowland areas became covered with a mantle 

 composed of irregular layers of stones of torrential origin, 

 together with banks of dry sand, transported, and frequently 

 re-arranged, by the action of the wind. In this way grew 

 up the Old Red Sandstone, which, in later periods, gradually 

 consolidated from the original loose state to the condition 

 of stone. 



After a time the old continent, with its mantle of desert- 

 formed sands, began a return movement to lower levels. As 

 a consequence of this subsidence, the sea coast-line gradually 

 drew nearer and nearer to these parts. The climate, from being 

 arid, changed by slow degrees to humid ; and, from the former 

 continental state, the geographical conditions, step by step, 

 assumed insular characters. Eain began to fall with more or 

 less regularity ; the diurnal range of temperature attained its 

 minimum, as did also the annual range; vegetable life began 

 to flourish, and with that change animals too began to thrive. 

 So set in the Carboniferous Period ; and for millions of years 

 afterwards, during a very slow, and somewhat intermittent, 



