184 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION. 



8. The gabbro and granophyre bosses of the Inner Hebrides demonstrate with singular 

 force how unreliable petrographical characters are as a test of the relative age of rocks. 

 No one looking at hand-specimens of these rocks, or even studying them in the field, 

 would at first suspect them to be of Tertiary date. They precisely resemble rocks of 

 similar kinds in Palaeozoic and even Archaean formations. Yet, of their late appearance 

 in geological time, there cannot be any possibility of doubt. 



9. After the uprise of the granophyre, and the injection of the network of felsitic 

 veins, there came once more a period of terrestrial convulsion like that of the earliest 

 basic dykes, but of less intensity. Again, the crust of the earth over the volcanic region 

 was pushed upward and rent open by another system of parallel fissures, directed in a 

 general N.N.W. line. Again, from a sea of basic lava underneath, molten rock was forced 

 upwards into the rents, and thus another system of basic dykes was formed. These dykes 

 are found crossing those of earlier date, and rising through the other volcanic rocks. They 

 traverse the plateau-basalts from bottom to top ; they climb to the summits of the gabbro 

 mountains, and they even pursue their undeviating course over the huge domes of 

 granophyre. No proof has yet been found that from any of these dykes there was a 

 superficial outflow of lava. But so great has been the subsequent denudation of the 

 areas, that such outflows might quite well have taken place, and have subsequently been 

 destroyed. 



10. Whether these basic dykes were the last manifestation of volcanic energy in our 

 region cannot yet be decidedly affirmed. But, so far as the evidence at present goes, 

 they are possibly older than another series of veins and dykes, consisting chiefly of 

 pitchstone, which are found at many points from Antrim to the far end of the Inner 

 Hebrides. These vitreous protrusions traverse every other member of the volcanic series, 

 and do not appear to be themselves cut by any. At one locality, the Scuir of Eigg, 

 they reached the surface, and flowed out in streams of molten rock over the basalt- 

 plateau, which had now been deeply trenched into valleys and ravines by running water. 

 The singularly durable pitchstone, flowing into a river-bed, has thus preserved an 

 impressive memorial of the vanished topography of Tertiary time, and of the enormous 

 duration of the Tertiary volcanic period in the geological history of the British Isles. 



