ANCIENT VOLCANIC ROCKS. 59 



and graduate at the edge into andesitic types ("porphy- 

 rites," etc.), which also form the smaller flows. The 

 diabases or dolerites have sometimes a granular, sometimes 

 an ophitic structure, and only very exceptionally contain 

 olivine. The felspar is sometimes labradorite, some- 

 times andesine, and in the latter case micropegmatite 

 is also present. The augitic porphyrites show every grada- 

 tion from holocrystalline to perfectly glassy rocks, and this 

 in two parallel series, one of ordinary andesitic types, the 

 other with a variety of radial, spherulitic and variolitic 

 structures. The thoroughly glassy type, which in at least 

 one flow retains its original character, is of oreat interest 

 from the extreme rarity of such rocks, except as narrow 

 selvao-es. The basic Qflass, often with scoriaceous and 

 pumiceous structures, is also an important constituent in 

 the fragmental volcanic accumulations associated with the 

 lavas. These occur abundantly except in connection with 

 the earliest outbreaks, in which the volcanic activity had 

 not yet reached the explosive stage. There are subaerial 

 tuffs, composed of more or less fine volcanic debris, enclos- 

 ing spheroidal scoriaceous bombs of andesitic material and 

 other fragments, and cemented by chlorite and other se- 

 condary products. Distinguished from these are stratified 

 submarine tuffs, sometimes fossiliferous, or again containing 

 pisolitic iron-ores not unlike those found in Antrim in 

 similar circumstances. There are also beds of breccia, 

 consisting of angular fragments of the porphyrites cemented 

 by calcareous matter. Barrois clearly shows in this memoir 

 how in the Menez-Hom district erosion has laid bare almost 

 the whole apparatus of vulcanicity in those early times. 

 The phenomena are in many respects comparable with those 

 of the Carboniferous in the basin of the Firth of Forth as 

 described by Sir A. Geikie (1879), except that "necks'' 

 marking the actual orifices of eruption have not yet been 

 discovered in the Finistere region. 



In America the recognition of the essential nature of the 

 older volcanic rocks has been retarded by various circum- 

 stances. An immense development of such rocks, chiefly 

 of pre-Cambrian age, exists along the eastern border of 



