PRE-CAMBRIAN ROCKS OF NEWFOUNDLAND 



463 



Crystal tuffs are of frequent occurrence interbedded with the 

 breccias and tuffs. They are remarkable, in view of their great age 

 and the vicissitudes which they have undergone, for the frequent 

 presence of a perfectly preserved vitroclastic structure in their 

 groundmass, whether rhyolitic or basaltic in nature. These 

 devitriiied glass shards are such a prominent feature of some of the 

 tuffs from the vicinity of Red Rock Lake near Brigus that they 



Fig. 9. — Microphotograph of vitroclastic groundmass of rhyolitic crystal tuff; 

 near Red Rock Lake, south of Brigus, Conception Bay. Natural light, Xs6 diam- 

 eters. 



might well be denominated vitric tuffs (Fig. 9). In other cases the 

 groundmass is an indistinct, microcrystalline, granular dust. 

 Many of these tuffs are soaked with hematite and hence are reddish 

 in color. Associated with the phenocryst-like crystals are frag- 

 ments of either porphyritic or felsitic basalt or rhyolite, or of 

 devitrified obsidian exhibiting trichites, microspherulites, spheru- 

 Htes, axiolites, perlitic cracks, pumiceous structure, or any combina- 

 tion of these. The crystals consist principally of plagioclase and 



