1 10 The Influence of Volcanic Action [Sess. 



cinder layers with occasional beds of sand, deposited in the 

 great midland lake during the peaceful epochs which inter- 

 vened between volcanic outbursts. To the south the Pent- 

 lands remind us of the widespread eruptions, as also do such 

 low-country peaks as Tinto Hill. On the coast line, too, we 

 find traces of their influence, for to the south of Stonehaven 

 and in the neighbourhood of Montrose consolidated lavas, 

 sometimes forming precipitous cliffs, occur. These are well 

 known on account of the " Scotch pebbles " or agates which 

 have formed within their steam cavities. 



Succeeding the " Old Eed " period of land-locked waters 

 came an age of swamps, when tree-ferns, giant horse-tails, 

 and huge club-mosses flourished, and, dying, created thick 

 beds of vegetable matter, which the pressure of succeeding 

 deposits converted into coal. Eruptions still continued dur- 

 ing this Coal Period ; and although their vigour had dimin- 

 ished, their effect upon our land surface is more varied than 

 that of any other time. Widely distributed through the 

 midlands, their remains are to be found from Kintyre to Had- 

 dingtonshire, sometimes rising in smooth-sided gentle cones, 

 such as the Eildon Hills in Tweed valley and the Law at 

 North Berwick ; again springing from sea or land with sudden 

 ruggedness, as in the Bass Eock, the castle rocks of Edin- 

 burgh, Stirling, and Dumbarton, or the precipitous Salisbury 

 Crags and Samson's Eibs. The diverse appearances are due 

 to the action of the weather upon the volcanic materials, and 

 are ultimately traceable to properties of the lavas or ashes 

 themselves ; for example, to some structural feature, such as 

 the well-marked joints which in Samson's Eibs determined the 

 directions in which rain and frost should split up the more 

 exposed portions, thus forming regular basaltic pillars. 



After the Carboniferous Period the eruptive energy decayed, 

 for only scattered relics, chiefly along the Eifeshire coast, 

 marked the positions of the volcanoes of Permian times. 

 Long ages of peace followed. Then in Tertiary times the 

 forces pent up within the earth burst their bounds, earth- 

 quakes shook the country, and lava welled up, as water wells 

 up through the cracked surface of an ice-covered lake, spread- 

 ing over the land in far-reaching sheets. This repeated over- 

 flowing resulted in the piling up of a lava desert which in 



