Tertiary Basalt-plateaux of North-western Europe. 145 



gabbros of St. Kilda, which display a considerable variety of texture 

 and composition and include basalts and dolerites. 



7. The author, having been able to visit St. Kilda, describes the 

 junction of the granophyre of that remote island with the basalts 

 and gabbros. He brought away a series of specimens and photo- 

 graphs which demonstrate that the acid rock has been injected into 

 the basic masses, traversing them in veins and enclosing angular 

 pieces of them. The granophyre is precisely like that of Skye and 

 Mull, and is traversed by veins of finer material, as in these islands. 

 "Where it has penetrated the basic rocks it sometimes forms a kind 

 of breccia-matrix in which the pieces of dark material are enclosed. 

 It has taken up a certain quantity of the basalt or gabbro, as 

 is shown by the abundant brown mica and hornblende which have 

 been developed in the acid rock, especially round the enclosed basalt- 

 fragments. The results of a microscopic examination of thin slices 

 of the rock at the junction are furnished by Mr. Harker. 



From Skye examples are given of triple dykes and sills, wherein 

 a central band consists of granophyre or spherulitic felsite, while 

 the two marginal bands are of basalt, diabase, or other basic 

 material. There does not appear to be any ascertainable connexion 

 between the acid and basic parts of such compound intrusions. In 

 some cases the basic, in others the acid portion is the older. 



8. By way of illustrating the probable history of the basaltic 

 plateaux of North-western Europe, the author gives a short 

 summary of the results of recent investigations of the modern 

 volcanic eruptions of Iceland, especially by Th. Thoroddsen and 

 A. Helland. He shows in how many ways the phenomena of that 

 island explain the facts which are met with in the study of our 

 Tertiary plateaux, and how, in some respects, the enormous 

 denudation of these plateaux throws light on parts of the 

 mechanism of the Icelandic volcanoes which are still buried under 

 the erupted material. 



9. Reference is made to the evidence of considerable terrestrial 

 movement since the Tertiary volcanic period, as shown by the 

 tilting of large sections of the plateaux in different directions, and 

 also by the existence of actual faults. Besides the normal faults, 

 which are not infrequent among the Western Isles, there occur 

 among the Faroe Islands instances of reversed faults, which 

 probably indicate disturbance of a more serious character. 



10. The concluding section of the paper deals with the effects of 

 denudation on the plateaux. With possibly some minor intervals 

 of partial depression, the present Tertiary volcanic tracts of the 

 British and Faroe Isles have remained as land ever since the volcanic 

 period. Their valleys were probably begun before the close of the 

 eruptions, and these hollows have been continuously widened and 

 deepened ever since. The result is a stupendous memorial of the 

 potency of the agents of geological waste. While the Inner 

 Hebrides abound in most impressive illustrations of this denudation, 



Phil Mag. S. 5. Vol. 41. No. 249. Feb. 1896. L 



