44 Reports and Proceedings — Geological Society of London. 



to have a wide distribution in Skye, and to occur also in Rum. 

 The remarkable alteration of the plateau-basalts as they approach 

 the gabbros of Loch Scavaig is referred to, and the microscopic 

 structure of these metamorphosed, rocks is described in notes supplied 

 to the author by Mr. A. Harker. An account is also given of the 

 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, especialty 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 connection 

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

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



(8) An account is given of the little islet of Hysgeir, about 

 18 miles to the west of the island of Eigg, which has been identified 

 by Dr. Heddle with the rock of the Souir of Eigg, and which the 

 author has visited in two successive years. The " pitchstone " is 

 precisely like that of the Scuir down to the most minute structure, 

 as is shown by an examination of the rock under the microscope 

 by Mr. Harker. There can be little doubt that this rock was 

 a superficial lava like that of Eigg, and there seems every 

 probability that it is really a westward continuation of the Scuir. 

 The Hysgeir pitchstone everywhere slips under the sea, so that its 

 bottom cannot be seen. 



(9) 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 wa3 r s 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. 



(10) Eeference is made to the evidence of considerable terrestrial 

 movement since the Tertiary volcanic period, as shown by the 



