318- Geikie — British Tertiary Volcanic Rocks. 



crumbled away, and the lava-currents that filled the river-bed, being 

 much harder than the surrounding rocks, were enabled in great 

 measure to resist the degradation. Hence the singular result now 

 appears that the former hills have been levelled down into slopes 

 and valleys, while the ancient valley occupies the highest ground 

 in the neighbourhood, and its lava-current stands up as the well 

 known precipitous ridge of the Scuir of Eigg. The gravel and drift- 

 wood of the old river are still to be seen under the rock of the Scuir. 



The author then proceeded to point out the possible connection 

 between these Tertiary volcanic rocks and the metamorphism of 

 different parts of the West Highlands. He showed that in Mull, 

 under Ben More, the volcanic rocks themselves give signs of having 

 been subjected to a process of metamorphism, and that they are 

 associated there with masses of syenite, like those of Eaasay and 

 Skye. MaccuUoch pointed out that the syenite of the two latter 

 islands was later than the Secondary rocks of that district ; and there 

 now seems to be a strong probability that it will turn OTit to be of 

 Miocene age. Parts of that syenite are true granite, while the Lias 

 around it has suffered an extensive metamorphism. It will be an 

 impoi-tant addition to our knowledge of the history of metamorphic 

 action, if the alteration of the Secondary rocks of the Hebrides is 

 eventually shown to be connected with the evolution of volcanic 

 rocks during the Miocene period. 



The wide extent to which the British Islands were affected by the 

 Miocene volcanos of the west was then referred to. That extent is 

 not to be measured by the area at present covered with Tertiary 

 volcanic rocks, nor even by the area which these rocks may have 

 originally overspread ; but from which subsequent denudation has 

 removed them. From the great volcanic ridge running through 

 Antrim and the Western Islands, -thousands of trap -dykes diverge 

 in a south-easterly direction. They become fewer as the distance 

 from that bank increases, yet they extend as far as the coast of 

 Yorkshire. No single dyke, indeed, has been traced across the 

 country from sea to sea ; but there can be little doubt that they all 

 belong to one series. They cut through all the formations up to and 

 including the Chalk, and they likewise traverse the older portions of 

 the Tertiary volcanic rocks. They must thus be of Tertiary age, and 

 belong to that series of igneous masses described in the present 

 paper. They do not usually run along lines of fault ; on the con- 

 trary, they are found to cross faults of fifty fathoms and upwards 

 without being deflected. Their evenness and parallelism show that 

 they must have ascended through fissures prepared for them by 

 subterranean movements. Thus we learn that in Tertiary times the 

 greater part of Scotland, the north of England, and the north of 

 Ireland, were cracked by earthquakes, and that liquid lava rose 

 through the hundreds of parallel rents, perhaps in some cases 

 actually reaching the surface. 



The last section of the paper was devoted to an account of the 

 denudation of the Tertiary volcanic rocks. It was shown that wide, 

 deep, and long valleys have been excavated out of the horizontal 



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