DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 137 



more crystalline sheets. These at once remind one of the altered basalts of Skye and 

 Mull. On the west side also, beds of basalt emerge from under the gabbro, but they 

 have been so veined and indurated by the granophyre of that district, that their relations 

 to the gabbro are somewhat obscured. If we could restore the lost portions of the 

 plateau, I believe we should find the gabbros of Eum resting on part of the volcanic 

 plateau, and some of the gabbro-beds prolonged as intrusive sheets between the beds of 

 basalt. 



(c) Ardnamurchan. — The promontory of Ardnamurchan reveals as clearly as the 

 flanks of the Cuillin Hills, though in a less imposing way, the relations of the gabbros 

 to the plateau-basalts. From the southern shore at Kilchoan to the northern shore at 

 Kilmory, bedded basalts, of the usual type, amygdaloidal and compact, weathering into 

 brown soil, may be followed along the eastern slopes of the hills, resting upon the 

 quartzites and schists of western Argyleshire. These rocks are a continuation of those 

 that cap the ridges further to the south-east and cross Loch Sunart into Morven. They 

 dip westwards, and followed upwards in that direction, they soon present the usual 

 marks of alteration. They weather with a white crust and become indurated and 

 splintery. Sheets of dolerite with many veins and dykes of basalt run between and 

 across them. Bands of gabbro make their appearance, and these, as we advance 

 westwards, increase in number and in coarseness of grain until this rock, in its 

 characteristically amorphous form, constitutes practically the whole of the promontory 

 from Meall nan Con to the lighthouse. Many admirable sections may be seen on the 

 coast- cliffs and in the rugged interior, showing how prone the gabbro in its central 

 structureless portions is to develop segregation-veins. Large crystals of its component 

 minerals run in bands or ribbons across the rock, and traces of a peculiar arrangement 

 may be found to which I shall refer in the following account of the similar rocks of Mull. 



(d) Mull. — In the island of Mull, the conclusions to which the geology of the other 

 volcanic districts leads us as to the position of the gabbros in the series of volcanic 

 phenomena, are confirmed and completed. The first geologist who appears to have 

 observed the relation of these rocks in that island was Jameson, who classed them 

 under the old name of " greenstone," including in the same designation rocks now termed 

 dolerites and gabbros. He ascended one of the hills above Loch Don, probably Mainnir 

 nam Fiadh (2483 feet), -which he found to consist of "strata of basalt and greenstone," 

 with some basalt-breccia or tuff and a capping of basalt. He speaks of the " singular 

 scorified-like aspect " of the weathered greenstone — a description which applies to some 

 of the coarser gabbro bands of that locality. But he appears to have recognised the 

 general bedded arrangement of the rocks up even to the summit of the hill.* 



It was not, however, until the visit of Professor Zirkel in 1868, that the true 

 petrographical characters of the gabbro of Mull were recognised. The same observer also 

 remarked that the rock is regularly interstratified with the basalt.t Professor Judd, 

 as already stated, has supposed the gabbros to be the deep-seated portion of the masses 



* Mineralogy of the Scottish Isles, i. p. 205. t Zeitsch. Deutsch. Geol. Gesellsck, xxiii. (1871) p. 58. 



