R. A. Daly — Mechanics of Igneous Intrusion. 121 



( topography by J. McEvoy ; geology by E. G. McConnell, 

 1896 ) and on titie writer's observations made in the same area, 

 1902. The heavy terrace-deposits of the Columbia Yalley 

 average nearly a mile in breadth and consequently the bound- 

 aries of the different formations are there not to be fixed with 

 absolute precision. On account of the thick cover of drift and 

 a dense forest, the same remark applies to the formation-limits 

 indicated in the remainder of the area. The errors are, how- 

 ever, known to be of an order that cannot affect the usefulness 

 of the map for present purposes. 



The great complexity of the structure and the high degree of 

 alteration characteristic of the rocks have put still greater dif- 

 ficulties in the waj^ of defining the formations. This is par- 

 ticularly true of the attempt to subdivide the old volcanic 

 rocks which form a large part of the area. There appear to 

 be at least two volcanic rock-groups. The older is the oldest 

 formation exposed in the area, consisting of hard, dark-colored, 

 slaty ash beds interstratified with thick bands of squeezed and 

 much altered basic, amygdaloidal lava-flows, and agglomerates, 

 and cut by irregular dikes and boss-like dioritic, gabbroitic and 

 peridotitic masses, which are probably genetically related to the 

 lavas. As yet the required palaeontological and structural 

 evidences as to the age and thicknesses of these rocks are 

 lacking. In this paper the whole group may be called simply 

 the Older Yolcanic Series ; the probabilities are, as suggested 

 by McConnell, that it belongs to the Carboniferous or at least 

 to the upper Paleozoic. 



Overlying these rocks is a much younger (probably early 

 Tertiary ) formation, which, within the limits of the map, is 

 purely volcanic — a group of greatly dislocated breccias, tuffs 

 and flows derived from basaltic and essexitic magmas. Since 

 this formation chiefly composes the mountains northwest, west 

 and south of Rossland, it may be called the '' Rossland Yolcanic 

 Series." There also appears on the map the eastern extremity 

 of the coarse-grained basic rock-mass marked " gabbro " on the 

 Trail sheet, and referred by McConnell to an origin contempo- 

 raneous with that of the Rossi and effusives, probably represent- 

 ing the deep-seated portion of the main conduit through which 

 these eruptions took place. This mass is highly variable and 

 has gabbroitic, monzonitic and essexitic facies. 



Both of these older series of rocks had been intensely folded, 

 crumpled, and, in places, crushed by mountain-building forces 

 before the intrusion of the great " Nelson batholith " took 

 place. That huge granitic body covers an area of more than 

 3000 square miles. Its extreme southern end enters the area 

 mapped in the form of a curved tongue narrowing to the south- 

 ward and terminating in that direction at Yiolin Lake (see fig. 2 ). 



