602 J.V.LEWIS ORIGIN OF PILLOW LAVAS 



ably of Lower Arenig age. A close jointed dark gray chert occurs between 

 the pillows. 



Pillow lavas from Anglesey were described by Blake-*^ as rock that 

 ^Sveathers into some beautiful large spheroids, the interstices between 

 which are tilled with radiating zeolites." The glasslike substance outside 

 of the spheroids was found to contain microlites of feldspar. The sphe- 

 roidal structure was considered an original product of cooling and only 

 emphasized by weathering, and the ordinary spheroidal weathering of 

 basalt was thought to be the same. Cole-^ showed that the glassy coatings 

 of the pillows at this locality are variolitic and bear the same relation to 

 the diabase as the devitrified crusts at Mont Genevre, running into crev- 

 ices and wrapping round the masses. "This appearance is due to its 

 having developed as a product of rapid cooling on the spheroidal surfaces 

 of a lava."' Silica has replaced much of the original rock, even the whole 

 interior of some of the spheroids, leaving unchanged tachylitic ,crusts 

 wrapping round masses of red jasper. 



"This silica, which some would certainly associate with the action of hot 

 springs, rising perhaps in pre-Cambrian times at the close of local volcanic 

 activity, has actually replaced the igneous rock by a red or purple-red jasper." 



The basic lavas of southeastern Anglesey have . been described by 

 Greenly-^ as 



"composed of ellipsoidal or spheroidal masses piled and pressed upon one 

 another as if they had been rolled over and over in a semicousolidated condi- 

 tion. . . . Sometimes smaller ellipsoids or pillowy masses fit into gentle 

 reentering curves in the sides of the larger ones, suggesting very vividly, hard 

 though they now are, the rolling and pressing against each other of pasty 

 yet individualized bodies. ( Footnote : The structure is evidently distinct from 

 ordinary spheroidal jointing produced after consolidation.)'' 



Yarioles are arranged in zones near and approximately parallel to the sur- 

 faces, as in the Mont Genevre rock. The associated red jasper is thought 

 to be of radiolarian origin. 



The remarkably cellular (amygdaloidal) rock that forms the peak and 

 southern declivity of Cader Idris, Merionethshire, is thus described by 

 Geikie :-« 



^ .1. F. Blake : On the Monian system of rocks. Quar. Jour. Geol. Soc. London, vol. 

 44, 1888, pp. 463-547. The microscopic structure of the older rocks of Anglesey. Rept. 

 Brit. Asso. Adv. Sci., 58th meeting. 1888, pp. 367-420. 



" G. A. J. Cole: The variolite of Ceryg Gwladys. Anglesey. Sci. Proc. Roy. Dublin 

 Soc, new ser., vol. vii, 1891-1892, pp. 112-120. 



28 Edward Greenly : The origin and associations of the jasper of southeastern Anglesey. 

 Quar. Jour, Geol. Soc. London, vol. 58, 1902. pp. 425-440. 



29 A. Geikie : Ancient Volcanoes of Great Britain. London, 1897, p. 184. 



