384 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



small mass of volcanic rock which has been interpreted by Mr. Brock as a true 

 neck, which is much later in date than the main body of massive lava and 

 pyroclastic material round about. To the latter only is the proposed name to 

 be applied. Mr. Brock's description of the group is best given in his own words 

 (Report for 1902, p. 97) :— 



'The older pyroclastic rocks and porphyrites are widespread; in fact 

 they are the commonest rocks in the district. 



' This series of rocks consists of green tuffs and volcanic conglomerates 

 and breccias, fine ash and mud beds, flows of green porphyrite, and probably 

 some interbedded limestones and argillites. The tuffs, conglomerates and 

 breccias consist of a mixture of pebbles and boulders of porphyrite material 

 with a great many fragments (probably a large proportion) of the rocks 

 through which the volcanics burst. Pebbles and boulders of limestone, 

 argillites, jasper and chert are common. Such of serpentine and old granite 

 and old conglomerates are much rarer. In form the pebbles and boulders 

 are "rounded, subangular, angular and of irregular and fantastic outline. 

 Sometimes they are somewhat sorted but often they are tumultuously 

 arranged (agglomeratic). Beds of mud, ash and tuff alternate rapidly with 

 coarse volcanic conglomerates and agglomerates. Sometimes the matrix 

 seems to be formed of porphyrite injected between the boulders. Lime- 

 stone, now crystalline, seems occasionally to have been interbanded with 

 them. It is often arenaceous, bands containing rounded sand grains and 

 pebbles alternating with pure limestone. The sand and pebbles are well 

 sorted and these arenaceous bands are sharply defined from the pure lime- 

 stone. The matrix of these bands is white crystalline limestone. Argillites 

 are also interbanded to a limited extent, although it is not always possible 

 to distinguish the volcanic muds from such sedimentary material. 



' The porphyrite seems to be a little later than most of the pyroclastic 

 rocks although some of it may be interbanded. Owing to the alteration in 

 these rocks through mountain-building processes and contact metamor- 

 phism, it is not possible to separate the porphyrites from the pyroclastic 

 rocks, on the map. The porphyrite is usually too highly altered to make 

 out its original character, but it seems to have been an augite-porphyrite 

 similar to that of the West Kootenay district. In places it is agglomeratic. 

 ' The great changes produced by mountain-building processes and 

 later igneous intrusions, make it difficult or impossible to discover the 

 history of these rocks. The first part of this period of volcanism seemed 

 to have been one of heavy explosions with periods of sedimentation, and 

 to have been followed by a period of more quiet lava flows. The amount 

 of material extruded must have been very great. 



' A very striking feature in these rocks is the way in which islands 

 or irregular masses of the older sedimentary rocks appear in them. In 

 part, these are included fragments, in part they may represent infolded 

 masses in truncated anticlines, or inequalities in the surface on which 

 this old volcanic series was deposited. Appressed anticlines and faults 



