^^^^' 55-] ^^ ^^^ REGION OF LOCH AWE. 479 



The western part of the area possesses likewise its own type 

 of sills. These belong to the porphy rite-group. While they 

 are met with over the whole of the western division of this region, 

 they are most abundant where they occur as intrusions in the 

 volcanic series of Lome. These volcanic rocks of andesitic type my 

 colleague, Mr. R. G. Sjraes, has shown by palaeontological evidence 

 to belong to the age of the Lower Old Eed Sandstone, and my 

 colleague, Mr. H. Kynaston, who has mapped these porphyrites in 

 Jarge numbers in the district between Loch Awe and Loch Etive, 

 considers that they represent the intrusive phases of these Lower Old 

 E-ed eruptions : he has gone fully into the question of these rocks 

 in the Annual Reports by the Director-General of the Geological 

 Survey for the years 1897 & 1898. The age of the quartz-porphyries 

 and porphyrites of Loch Fyne cannot be so accurately determined. 



Besides these groups of porphyrites and quartz-porphyries, 

 which are fairly definite in character, there is another set 

 of intrusions of extremely variable composition, but agreeino- 

 more or less in their mode of occurrence, which have been classed 

 together in the rather unsatisfactory group of lamprophyres. 

 They usually occur as sills, and include rocks of acid and basic 

 type. In this area the bulk of them are basic, but the group also 

 contains rocks of dioritic character, and varieties of porphyrites as 

 well as rocks which are allied to hyperites. Rocks of this kind 

 are found over the entire district to which this paper refers, but 

 they are most numerous in its northern parts. The typical 

 dark basic lamprophyres form a big group in Glen Shira, where 

 they occur as a set of sills parallel to the porphyrite-group of 

 Glen Fyne, and occupying an horizon of very similar extent. It is 

 noteworthy, too, that they come on in large numbers just where th;^ 

 quartz-porphyry group dies out, and they appear to play the same 

 part in Glen Shira that the quartz-porphyries do to the south, and 

 the porphyrites to the east. The lamprophyres of Glen Shira 

 would appear to be later than the quartz -porphyries, which they 

 replace, there being at least one clear instance of a lamprophyre 

 intruded into a quartz-porphyry. 



The porphyrites, quartz-porphyries, and lamprophyres have been 

 cut by a set of basalt- and dolerite-dykes. These are of 

 two types — the east-and-west type, which is rare, and the north- 

 west type, which is not only the later but is the common form of 

 dyke in. the area. These latter are so numerous that they 

 literally occur in hundreds. While they are prevalent over the 

 entire region, they are most numerous in the great central belt 

 which traverses the southern half of Loch Awe and the valley 

 of Kilmartin beyond. They vary in character from coarse 

 dolerites to the finest-textured basalts, and in composition from 

 augite-andesites or andesitic basalts to olivine-basalts. They have 

 been fully described by Messrs. Gunn, Clough, and myself in the 

 Survey memoir on Cowal, and by Sir A. Geikie,^ who considers 

 them to be related to the Tertiary volcanic rocks of Mull. 



^ ' The History of Volcanic Action during the Tertiary Period in the British 

 Isles,' Trans. Eoy. Soc. Edin. vol. xxxv (1890). 



