AXN'IVEKSAKY ADDRESS OF THE PRESIDENT. 1 33 



(e) Acid rocks, such as Felsite and Quartz- porphiry, as I have 

 already said, are extremely rare among the puy-eruptions. The 

 only important examples known to me are those around the Limerick 

 basin, where they rise apparently in old vents and form con- 

 spicuous rounded or conical hills. These rocks have recently been 

 examined microscopically by Mr. W. W. Watts. One of the most 

 interesting varieties, which occurs at the Standing Stone near Oola, 

 was found by him to show quartz enclosing ophitically the felspars 

 which, with well-terminated prisms, project into it. Farther west, 

 near Knockaunavoher, a boss of quartz-porphyry occurs with con- 

 spicuous quartz. 



iii. Tuffs. — The fragmental rocks connected with the puy-erup- 

 tions form a well-marked group, easily distinguishable, for the 

 most part, from the tuffs of the Plateaux. They vary from ex- 

 ceedingly fine compacted dust or volcanic mud, through various 

 stages of increasing coarseness of texture, to basalt-conglomerates 

 and tumultuous agglomerates. 



Of the finer kinds, the best example is furnished by a remarkable 

 group of ' green and red marls ' which lie above a seam of coal 

 (Houston Coal) in the Calciferous Sandstones of West Lothian.^ 

 These strata, which differ much from any of the rocks with which 

 they are associated, are exceedingly fine in grain, not well lami- 

 nated like the shales, but breaking into irregular fragments under 

 the influence of weathering. They look like indurated mud. 

 Mr. H. M. Cadell, who has recently re-examined them in connexion 

 with a revision of the Geological Survey Map (Sheet 32), has found 

 that they pass into ordinary granular tuff. They appear to mark a 

 phase in the volcanic history of the Lower Carboniferous rocks of the 

 Firth of Forth, when exceedingly fine ash or perhaps even volcanic 

 mud was erupted in considerable quantity. The ' marls ' attain in 

 some places a thickness of nearly 200 feet, and can be traced over 

 most of the eastern part of Linlithgowshire. This volcanic plat- 

 form, which has been followed in mining for oil-shale, is one of the 

 most extensive among the puy-eruptions. The material probably 

 came from one or more vents among the Bathgate Hills. 



Palagonite-tuff occasionally occurs. It is met with in the 

 Burntisland district/ and Mr. Watts has detected fragments of 

 palagonite among the tuffs of the Limerick basin. 



Basalt-tuff and basalt-conglomerate are the usual forms of frag- 



1 Memoir on Sheet 32, Geol. Surv. Scotland (1861), p. 42. 



2 Trans. Roy. See. Edin. vol. xxix. (1879) p. 515. 



TOI. XLVITI. Tc 



