342 SIR A. GEIKIE ON THE TERTIARY [May l8o,6 r 



sandy stratum is only some 3 or 4 inches thick at the northern 

 end of the section, but increases rapidly southward to a thickness 

 of as many feet or more, when, owing to the cessation of the 

 underlying shale, it comes to lie directly on the amygdaloid and to 

 enclose slaggy portions of that rock. Immediately above the sand- 

 stone 2 or 3 feet of fissile shale, black with plant-remains (d), 

 include brown layers that yield to the knife like some oil-shales. 

 The next stratum is a seam of coal (e) about 1 foot thick, of re- 

 markable purity. It is glossy, hard, and cubical, including layers 

 that break like jet. It has been succeeded by a deposit of green 

 sand (/), but while this material was in course of deposition 

 another outpouring of lava took place, whereby the terrestrial pool 

 or hollow in the lava-field in which the group of sedimentary 

 materials accumulated was filled up and buried. This lava is 

 about 20 feet thick, and consists of a coarsely-crystalline, jointed 

 dolerite with highly amygdaloidal upper and under surfaces. Its 

 slaggy bottom has caught up or pushed aside the layer of green 

 sand, so as to lie directly on the coal, and has there been converted 

 into that earthy modification so familiar under the name of ' white 

 trap ' among our coal-fields. It is interesting to find that this kind 

 of alteration, where molten rock comes in contact with carbonaceous 

 materials, is not confined to subterranean sills, but may show itself 

 in lavas that have flowed over a terrestrial surface. 



Prom the frequent intercalation of such local deposits of sedimen- 

 tary material between the basalts we may reasonably infer that 

 during older Tertiary time the rainfall in North-western Europe 

 was copious enough to supply many little lakes and streams of 

 water. As the surface of the lava-fields decayed into soil, vegeta- 

 tion spread over it, so that perhaps for long intervals some tracts 

 remained green and forest- clad. But volcanic action still continued 

 to show itself, now from one vent, now from another, these wooded 

 tracts were buried under overflows of lava, and, the watercourses 

 being filled up, their streams were driven into new channels," and 

 other pools and lakes were formed. Some of the evidence for this 

 part of the volcanic history will be given in the Illrd section of the 

 present paper. 



II. The Tents. 



Though the abundant vents, which, to judge from the lenticular 

 nature of the lavas, were dotted over the surface of the Tertiary 

 volcanic plains, have for the most part been buried under sheets of 

 molten material, the progress of denudationhas laid bare some of them. 

 It is chiefly along the coast-line that this process of excavation has- 

 successfully taken place. The interior of the islands is often loaded 

 with peat, covered with herbage, or strewn with glacial detritus ;: 

 and even where indications of the vents are there to be detected, 

 it is not always possible to ascertain the true limits and connexions 

 of these old volcanic chimneys. But where the structure of the 

 plateaux has been laid bare along ranges of rocky precipice, the 

 vents have sometimes been so admirably dissected by the sea that 



