IGNEOUS CORES. 397 



lavas and breccias whicli form the eastward extension of the range ; it has 

 also a small number of radial dil^es. Tlic accompanying figure, 2, pre- 

 sents a nearly east and-wcst section througli Comb butte and Lava peak. 

 It cuts the soutli and east cores and sliows the accumulations of extrusive 

 material resting on the horizontal Cretaceous strata. 



Shonkin Core. — The northern half of the mountains is of lower general 

 elevation tlian that to the south, and it is drained by the branches of 

 Shonkin creek. Seen from a. distance, the area is one of convex sum- 

 mits and gently contoured sloi)es, devoid of timber, but covered with a 

 velvety naj) of grass, giving the smooth, rounded efiect of a topograjihic 

 model, ^^'hen visited this area is found to l)e dce[)ly trenched ])y sharp 

 V-shaped gorges, whose streams flow in rock-cut channels, which expose 

 excellent sections of the geologic structure. On the west the long north- 

 west s[)ur of the mountains, terminating in Twin })eaks, divides the waters 

 of Highwood creek from Shonkin. This ridge shows only the dark basaltic 

 lavas, generally fragmental, and similar to those which compose Arrow 

 peak and the adjacent ridges. To the northeast outliers of the same 

 ])asaltic lavas and l)reccias form mountain masses resting upon sedi- 

 mentary ridges. 



Tiie valleys of the numerous branches of Shonkin creek are largely cut 

 in sedimentary strata, which are everywhere penetrated by dikes whose 

 parallel trend makes their dark walls, rising above the grassy slopes, all 

 the more impressive. 



The multitude of dikes in the foothills and benchland north of the 

 mountains has already been noted. From Palisade butte westward to 

 Twin peaks these dikes radiate from an area which examination proved 

 to be the largest massif of the grou}). IJadial dikes also occur in large 

 numbers on the south and west of this center, but are less conspicuous and 

 are a.ssociated with some which belong to the Highwood Peak system. 



The Shonkin core is the largest one of the group. Denudation has as 

 yet but partially stripped it of its covering of tuffs and l)reccias. The 

 granular rock is, however, well exposed in the creek bed and the steep 

 slopes on each side. This rock has broken through horizontal beds of 

 sandstone and shale, which are greatly altered by contact metamorphism. 

 An offshoot of the main body extends southwest under a cover of these 

 altered l>eds to the slopes drained by Highwood creek. The streams 

 which trench the slopes expose the massive rockjwiiich is also seen on 

 the crest between thu two drainages. At the latter place acidic tuils and 

 breccia,s rest on the sedimentary beds, and are at times very ditlicult to 

 distinguish from them. The main body of igneous rock, as well as this 

 ott'shoot, shows in its rapid variations considerable differentiation, though 

 not so much a.s at Highwood peak. The rock usually weathers into 



LVII— Btri.i.. (iroi.. 8or. Am., Vol. 6, 1894. 



