548 STKATIGKAPHICAL GEOLOGY 



Feet. 

 Rudely columnar basalt .... 10 



Bedded sandstone 

 Indurated gravel of flints and lava-fragments 

 Indurated dark-coloured mud with ferns 

 Black crumbly shale full of leaves . 

 Hard gravelly sand resting on basalt 



8 



7 



1 



24 



2 



At other points there are several beds of clay or shale which 

 contain leaves, and similar beds also occur near Carsaig. The 

 plant remains include a fern (Onoclea hebridica) and an Equisetum, 

 many Gymnosperms, such as GinJcgo, Podocarpus, Taxus, and Sequoia. 

 Dicotyledons are abundant and include leaves of Platanus hebridicus, 

 Populus arctica, Cornus hyperborea, Boehmeria antiqua, and leaves 

 like those of Oorylus, Laurus, and Rhamnus. Mr. Gardner 

 considered the flora to be more like a late Cretaceous than a 

 Tertiary one, but accepting its probable connection with the Antrim 

 area, was inclined to regard it as of very early Eocene age, and not 

 newer than the time of the Thanet Beds. 



That the island of Arran was included in the area of Tertiary 

 volcanic activity has long been suspected, but it was only in 1899 

 that absolute proof of the fact was obtained. 13 No remnants of 

 basaltic lava-flows remain on the island, but all the southern part 

 of it is seamed with basalt dykes, and it is very probable that 

 many of these belong to the earlier dyke-system, and that surface 

 eruptions proceeded from them ; all traces of such eruptions have, 

 however, been destroyed. 



The most remarkable feature of the island is the large volcanic 

 vent which occupies the central district around Glen Oaigagh, 

 between Brodick and Machrie Bays. The complex of rocks by 

 which this vent is now filled has an oval form about 3| miles 

 in length by 3 in breadth ; its central portion (over 2 miles in 

 diameter) is composed of a breccia or agglomerate, consisting 

 largely of fragments of sedimentary rocks mixed with a smaller 

 quantity of igneous rock-fragments. This area is surrounded by a 

 ring of intrusive igneous rocks, which are chiefly granites, grano- 

 phyres, and felspathic rocks. 



Some of the rock-fragments in the central agglomerate are Old 

 Red Sandstone, and have been blown up from below; others are 

 large blocks of Trias, Rhaetic, Lias, and Chalk, which must have 

 fallen into the vent from above, and testify to the existence of a 

 former covering of Jurassic and Cretaceous strata of which all 

 other traces have been destroyed. As remarked by Sir A. Geikie 

 in the Summary of Progress for 1900 (p. 124), "the explosion was 

 thus certainly later than the Upper Cretaceous period and may 

 consequently be referred to older Tertiary time. The fixing of the 



