Ancient igneous RocJiS. 61 



eye this rock consists of a mixture of white plagioclase and glassy 

 quartz, in which is imbedded occasional prisms of hornblende 

 which seldom have a length of less than a quarter of an inch ; 

 occasional plates of biotite also occur. The contact of this rock 

 with the slates is best shown on the northern wall of the valley 

 of the small glacier Avhich lies north of Pyramid peak and was 

 formerly tributary to Dying glacier. An angle in the wall cuts 

 through the slates into the quartz-diorite, showing two contacts. 

 The westerly one is sharp and nearly vertical. The eastern one 

 is inclined toAvard the west, and numerous stringers are seen 

 penetrating the slates. This indicates the more recent date of 

 the eruptive rock. The slates in the near vicinity are apparently 

 not greatly affected by the heat consequent on this outfloAV, but 

 the same holds true of them when in juxtaposition to the more 

 recent dikes, and is accounted for l^y the metamorphosed condi- 

 tion of the Avhole series. 



Three small exposures of rock, clearly distinguishal^le from 

 the main quartz-diorite mass raacroscopically, but Avhich Dr 

 Williams also describes as quartz-diorite, occur. One of these 

 is on the most northerly spur of mount Wright, at Mr Reid's 

 station E ; another forms the southern jn'ojection of the ridge 

 west of Granite canyon ; the third is found cutting the limestone 

 on the shore of the bay 10 miles south of Muir glacier. Their 

 relations to the main mass could not be determined. They are 

 easily distinguishable from the main mass, which is very uniform 

 in appearance, by their greater percentage of hornblende and 

 their metalliferous contents. Their contacts Avith the adjoining- 

 rocks clearly indicate their outfloAV to be of later date than the 

 deposition of those rocks. 



These quartz-diorites are clearly quite old, and have suffered 

 equally Avith the clastic rocks from the disturbances Avhich the 

 region has undergone. They are cut by the same sets of fissures, 

 though not so numerously ; they are also cut l)y the more recent 

 dikes. Dr Williams strongly urges the ancient date of these 

 diorite masses from his microscopic examination. This is cor- 

 rol:)orated by their appearance in the field and strengthens the 

 meager fossil evidence obtained as to the early age of the clastic 

 rocks. 



Later Eruptives. — All the rocks previously described are cut by 

 numerous small dikes of later date. The key to their arrange- 

 ]nent Avas not apparent. They seem to lie in all possible atti- 



