Yol. 57.] INTKTJSTVE TUFF-LIKE ROCKS IN IRELAND. 



487 



origin of fragmental igneous masses. Pig. 8 shows a view of the- 

 mainland and the northern end of Sheep Island. Here are seen the 

 light buff-coloured and green felsites, which in parts present a tuif- 

 like aspect, and are described by Mr. Cowper Reed as tufiPs.^ The 

 green rock has invaded the light-grey felsite and limestone, which 

 it has in parts marmorized and in parts thoroughl}' impregnated 

 with green (? chloritic) matter, as appears in the small islet in 

 the figure. Though the green felsitic rock sends veins into the 

 adjoining felsite, as seen in the accompanying figure and at the east 

 side of the spur on the mainland near the islet (as shown in Sir 

 Archibald Geikie's sketch),^ the line of demarcation between the two 

 felsites on the western side of the spur is not so obvious ; there 

 seems, in fact, to be a gradation (though across a line of fracture) 



Fig. 8. — y"ie20 of Sheep Island 2^'>'omontor>f, soutli- west of Tr amove 



(County Waterford). 



[Length of section = 50 yards.] 

 L=:Llandeilo black slate. T= Pinkish-grey felsite, 1 both in parts 

 B=Bala liraestone. F=Dark green felsite, j tuflf-Hke. 



from the green fragmental mass into broken-up, light-grey felsite, 

 which contains a vanishing amount of green matter from the later 

 rock as a cemen ting-matrix. Specimens from these two fragmental 

 varieties have been microscopically examined and found to contain 

 hourglass lapilli, indicative of tufaceous structure. 



Mr. H. J. iSeymour has ingeniously suggested'* a method of 

 accounting for such lapilli in intrusive rocks, namely, that they 

 represent the glassy interspaces between spherules in rapidly-cooled 

 masses which were mechanically fractured by pressure after cooling. 



1 Quart. Journ. Geol. Soc. vol. Ivi (1900) p. 663. 



'^ ' Summaiy of Progress of Geol. Siirv. U. K.' for 1809, p. 80. 



3 Ibid. pp. 179, 180. 



