CHAPTER XXVI 
THE CARBONIFEROUS PUYS OP SCOTLAND 
i. General Character and Distribution of the Pays; ii. Nature of the Materials 
Erupted Lavas Ejected at the Surface — Intrusive Sheets — Necks and Dvkes— 
Tuffs. ■' 
i. GENERAL CHARACTER AND DISTRIBUTION 
AI'IER the beginning of the Carboniferous Limestone period, wlien eruptions 
of the plateau-type had generally ceased, volcanic activity showed itself over 
the area of the British Isles in a different guise both as regards the nature 
of its products and the manner and scale of their discharge. Instead of 
widely extended lava-sheets and tuffs, piled above each other sometimes to a 
tliickness of many hundred feet, and stretching over hundreds of square 
miles, we liave now to study the records of another phase of volcanism, 
where sca,ttered groups and rows of Puys, or small volcanic cones, threw out 
in most instances merely tuffs, and these often only in trifling quantity 
though here and there their vents also poured forth lavas and gradually 
piled up volcanic ridges which, in a few cases, almost rivalled some of the 
plateaux. The evidence for these less vigorous manifestations of volcanic 
activity is furnished (1) by layers of tuff and sheets of basaltic-lavas inter- 
calated among the strata that were being deposited at the time of the 
eruptions, (2) by necks of tuff, agglomerate, or different lava-form rocks that 
mark the positions of the oriflces of discharge, and (3) by sills, bosses, and 
dykes that indicate the subterranean efforts of the volcanoes. The com- 
paratively small thickness of the accumulations usually formed by these 
vents, their extremely local character, the numerous distinct horizons on 
which they appear, and the intimate way in which they mingle and alternate 
with the ordinary Carboniferous strata are features which at once arrest the 
attention of the geologist, presenting, as they do, so striking a contrast to 
those of the plateaux. 
From the clear intercalation of these volcanic materials on successive 
platforms of the Carboniferous system, the limits of geological time within 
which they were erupted can be fixed with considerable precision. It may 
be said that, in a broad sense, they coincided with the period of the 
Carboniferous Limestone, and certainly it was during the depo.sition of that 
