annivi:rsaey address of the president. 143 



the superficial eruptions were especially vigorous and prolonged. 

 It would seem from these facts that the extent to which the crust 

 of the earth round a volcanic orifice is injected with molten rock 

 in the form of intrusive sheets between the strata does not depend 

 upon the energy of the volcano as gauged by its superficial out- 

 pourings, but on other considerations not quite apparent. Possibly, 

 the more effectively this volcanic energy succeeded in expelling the 

 materials from the vent, the less opportunity was afforded for sub- 

 terranean injections. And if the protrusion of the sills took place 

 after the vents were solidly sealed up with agglomerate or lava, it 

 would doubtless often be easier for the impelled magTaa to open a 

 way for itself laterally between the bedding-planes of the strata 

 than vertically through the thick solid crust. The size and extent 

 of the sills may thus be a record of the intensity of this latest 

 phase of the volcanic eruptions. 



Dykes take a comparatively unimportant place in the eruptive 

 phenomena of the puys. They occur in some numbers, but on a 

 small scale, among the tuff vents, and there they can without much 

 hesitation be set down as part of the material that was discharged 

 through these pipes. It is more difficult to ascertain the age of the 

 dykes which traverse the Carboniferous rocks at a distance from 

 the vents. I have given reasons for classing the east and west 

 dykes as probably Tertiary.^ Others may be Permian ; while a 

 certain proportion may, with some probability, be regarded as parts 

 of the puy-eruptions of Carboniferous time. The materials of these 

 dykes resemble those of the finer-grained sills. They are chiefly 

 dolerites and olivine-basalts (diabases). 



IV. Illustrative Examples of Puy-ekfpticns. — A better idea 

 of this interesting type of Carboniferous volcanic action will pro- 

 bably be obtained from a brief account of a few districts where it is 

 typically developed. I have elsewhere dealt so fully with the 

 basin of the Firth of Forth that it will be enough to refer to 

 published papers on that region.^ A less known district is that of 

 the north of Ayrshire. I have had occasion to allude to the 

 marked band of volcanic materials which there intervenes between 

 the Carboniferous Limestone and the Coal Measures, and from its 

 position appears to mark the latest Carboniferous outflows of 



1 Trans. Roy. Soc. Edin. vol. xxxv. (1888) pp. 29 et seq. 



'' See especially Maclaren's ' Geology of Fife and the Lothians,' the Memoirs 

 of the Geological Survey of Scotland, on Sheets 31 and 32, and my Memoir, 

 already cited, Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 437. 



