DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 183 



between the successive sheets of basalt, the leaves, stems, and fruit of land-plants, some- 

 times in most perfect preservation, may be observed, together with the remains of 

 insects. It is remarkable that the volcanic discharges consisted mostly of lava. 

 Fragmentary materials were comparatively insignificant in amount, and local in origin, 

 though layers of fine tuff and basalt- breccias occur in all the plateaux. Neither these 

 materials nor the lavas thicken towards any centres that might be taken to mark 

 volcanoes of the type of Vesuvius or Etna. On the contrary, the persistent flatness and 

 uniformity of the volcanic series, and the thinning out of the separate beds in different 

 directions, point to the existence of many minor vents from which the discharges took 

 place. The positions of not a few of these vents can still be ascertained. They are now 

 filled sometimes with dolerite, sometimes with coarse agglomerate. As the pile of erupted 

 volcanic materials of the plateaux gradually thickened, and the subterranean energy grew 

 feebler, the ascending lava, instead of rising to the surface, was forced between the layers 

 of sedimentary strata underneath or between these and the overlying basalts, so as to 

 form intrusive sheets or sills. 



6. When the great plateaux of basalt had been built up to a thickness of several 

 thousand feet, another remarkable episode in the volcanic history occurred. This con- 

 sisted in the uprise at certain points of coarsely crystalline basic rocks, which ultimately 

 solidified as dolerites, gabbros, troctolites, picrites, &c. There is reason to believe that the 

 points of extravasation of these materials were mainly determined by the positions of the 

 larger or more closely clustered vents of the plateau-period, where lines of weakness 

 consequently existed in the terrestrial crust. Rising as huge bosses through such weak 

 places, the gabbros and associated rocks raised up the overlying bedded basalts, and forced 

 themselves between them, forming thus a fringe of finer-grained intrusive sills and veins 

 around the central amorphous cores of more coarsely crystalline material. Whether, in 

 any of these vast domes of upheaval, the summit was disrupted, so as to allow the basic 

 intrusion to flow out as lava at the surface, cannot now be told, owing to enormous 

 subsequent denudation. 



7. The next chapter in the chronicle shows us that probably long after the eruption 

 of the gabbros, when possibly all outward symptom of volcanic action had ceased, a 

 renewed outbreak of subterranean activity gave rise to the protrusion of another and 

 wholly different class of materials. This time the rocks were of a markedly acid type. 

 They included varieties that range from dark flinty felsites through porphyries and 

 granophyres into compounds which cannot be classed under any other name than granite. 

 These masses likewise availed themselves of older vents in the plateaux, and broke through 

 them. They now form huge conical hills, which, in their outer aspect, and even to some 

 extent in their inner structure, recall the trachytic puys of Auvergne. But the grano- 

 phyres not only ascended through the basalt-plateaux and the gabbro-bosses ; they sent 

 into these rocks a network of veins, and pushed their way in huge sheets or sills between 

 the strata below. Around the bosses of gabbro and granophyre, the bedded basalts 

 have undergone considerable contact-metamorphism. 



