76 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



basalt-plateaux. So far as we know, these outbursts were subserial. At least no trace 

 of any marine deposit has yet been found even at the base of the pile of volcanic rocks. 

 Sheet after sheet of lava was poured out, until several thousand feet had accumulated, 

 so as perhaps to fill up the whole depression, and once more to change entirely the aspect 

 of the region. But the Volcanic period, long and important as it was in the geological 

 history of the country, came to an end. It too was merely an episode during which 

 denudation still continued active, and since which subterranean disturbance and superficial 

 erosion have again transformed the topography. In wandering over these ancient lava- 

 fields, we see on every hand the most stupendous evidence of change. They have been 

 dislocated by faults, sometimes with a displacement of hundreds of feet, and have been 

 hollowed out into deep and wide valleys and arms of the sea. Their piles of solid rock, 

 thousands of feet thick, have been totally stripped off from wide tracts of ground which 

 were once undoubtedly buried under them. Hence, late though the volcanic events are 

 in the history of the land, they are already separated from us by so vast an interval that 

 there has been time for cutting down the wide plateaux of basalt into a series of mere 

 scattered fragments. But the process of land-sculpture has been of the utmost service to 

 geology, for, by laying bare the inner structure of these plateaux, it has provided 

 materials of almost unequalled value and extent for the study of one type of volcanic 

 action. 



§ 1. Petrography. 



The superficial outbursts of volcanic action during Tertiary time in Britain are 

 represented by a comparatively small variety of rocks. By far the largest area and 

 thickest mass consist of dark basic lavas. In only one locality (Isle of Eigg) has any 

 outflow of acid lava been detected. Between the lava-sheets occasional layers of volcanic 

 and even non-volcanic fragmental rocks occur. The general lithological characters of 

 the whole group of plateau-rocks may here be briefly enumerated. 



1. Lavas. — In external characters these rocks range from coarsely crystalline varieties, 

 in which the constituent minerals may be more or less readily detected with the naked eye 

 or a field-lens, to dense black compounds in which only a few porphyritic crystals may 

 be microscopically visible. They are easily recognised as pertaining in the vast majority 

 of cases to the great group of the dolerites and basalts. One of their characteristic 

 features is the presence of the ophitic structure, sometimes only feebly developed, some- 

 times showing itself in great perfection. Many of the rocks are holo-crystalline, but 

 usually show more or less interstitial matter (dolerites) ; in others the texture is finer, 

 and the interstitial matter more developed (basalts) ; in no case, so far as I have observed, 

 are there any glassy varieties, which are restricted to the dykes, though in some of the 

 basalts there is a considerable proportion of glassy or incompletely devitrified substance. 

 The felspars are of the characteristic lath-shaped forms, and are usually quite clear and 

 fresh. The augite resembles that of the dykes, occurring sometimes in large plates that 

 enclose the felspars, at other times in a finely granular form. Olivine is frequently not 



