Presidential Address to Geological Section. 469 



found the oolitic iron-ore in the Middle Lias of Raasay, the position of 

 which corresponds approximately with that of the Cleveland ironstone.' 



The extensive plateau of Tertiary volcanic rocks in the Inner Hebrides 

 has been a favourite field of research ever since the time of Macculloch, 

 the great pioneer in West Highland geology. During the period under 

 review much work has been done in that domain. According to Professor 

 Judd, that region contains the relics of five great extinct volcanoes and 

 several minor cones, indicating three periods of igneous activity. The 

 first was characterized by the discharge of acid lavas and ashes, the molten 

 material consolidating down below as granite ; the second by the outburst 

 of basic lavas, now forming the basaltic plateau, connected with deep- 

 seated masses that appear now as gabbro and dolerite ; the third by the 

 appearance of sporadic cones, from which issued minor streams of lava.^ 



In 1888 Sir A. Geikie communicated his elaborate monograph on the 

 history of Tertiary volcanic action in Britain to the Royal Society of 

 Edinburgh,' which has been incorporated, with fuller details, in his recent 

 work on " The Ancient Volcanoes of Great Britain." His main conclusions 

 may thus be briefly stated : (1) The great basaltic plateaux did not emanate 

 from central volcanoes, but are probably due to fissure ei'uptions ; (2) the 

 basaltic lavas were subsequently pierced by laccolitic masses of gabbro, 

 which produced a certain amount of contact alteration on the previously 

 erupted lavas ; (3) the protrusion of masses of granophyre and other acid 

 materials by means of which the basic rocks were disrupted. 



During the last six years Mr. Harker has been engaged in mapping the 

 central part of the Isle of Skye and in the petrographical study of the 

 rocks, the results of which have been summarized in the annual reports 

 of the Geological Survey. As regards the basaltic lavas, he finds that 

 while they have been of vast extent the individual flows have been of 

 feeble volume, and show no evident relation to definite centres of eruption. 

 There were two local episodes, however, which took the form of central 

 eruptions : one represented by a number of explosive outbursts at certain 

 points ; the other, in the basalt succession, gave rise to rhyolitic rocks. 



Mr. Harker further finds that the succeeding plutonic phase of activity, 

 confined in Skye to what is now the central mountain tract, is represented 

 by three groups of plutonic intrusions, in the following order : peridotites, 

 gabbros, and granites. The metamorphism set up in the basaltic lavas 

 near the large plutonic masses presents points of interest, especially 

 the widespread formation of new lime-soda-felspars from the zeolites in 

 the lavas. 



After the intrusion of the granite of the Red Hills, Mr. Harker finds 

 that igneous activity took the form of intrusions of smaller volume, but 

 in some cases of wide distribution. The great group of dolerite sills 

 belongs to this period. An enormous number of acid and basic dykes 

 followed, of several distinct epochs. A set of minor basic intrusions 

 of quite late date is found in the gabbro district of the Cuillins, the 

 most interesting of which takes the form of sheets of dolerite, parallel at 

 any given locality, but always dijiping towards the centre of the gabbro 

 area. Mr. Harker considers that this remarkable system of injections 

 presents a new problem in the mechanics of igneous intrusion. The latest 

 phase of vulcanicity in the Cuillin district is a radial group of peridotite 

 dykes. As regards the local group of rock in Central Skye Mr. Harker 

 finds that the order of increasing acidity which ruled in the plutonic 

 phase was reversed for the minor intrusions which followed. 



In connection with the great development of volcanic activity in the 



1 Geol. Mag., Dec. Ill, Vol. X (1893), p. 493. 



2 Quart. Journ. Geol. Soc, vol. xxx, p. 220. 



3 Trans. Eoy. Soc. Edinb., vol. xxxv, pt. 2, p. 23. 



