718 BESCRIPTIVE GEOLOGY. 



often in situ, they have distinctly the characteristics of atrue quartz-porphyry, 

 but from their gradations into an argillaceous and arenaceous sandstone it 

 seems clear that the extreme products are only phases of metamorphism. 

 Directly under these beds, and also conformable with them and the lime- 

 stones, occur heavily-bedded quartzites, which are very variable in compo- 

 sition, the same bed changing on its strike from a fine-grained micro- 

 crystalline quartzite to an argillaceous schist, and passing on into a coarse 

 granular condition. Up to the region of the anticlinal axis, the structure 

 of these argillaceous and quartzitic rocks is clearly conformable with the 

 upper limestone, but, as they approach the axis, the planes of sedimentation 

 become more and more obscure, the porphyritic character of the metamor- 

 phism more and more highly developed, until finally all structure-lines are 

 obliterated, and the formation possesses many of the characteristics of an 

 eruptive porphyry. Between the granite and the limestone in Wright's 

 Caiion, the zone of porphyroids reaches 700 feet in thickness. 



In the large canon next north of Wright's Cafion, the limestone wraps 

 around the western margin of the granite, and stands quite vertical. Near 

 this locality is a somewhat obscure series of beds of quartzite, which, near the 

 granite, is covered by debris. This quartzite, from its fine-bedded and fissile 

 character, as well as frojm its position in the limestone, is referred to the 

 lowest zone of quartzite, which overlies the lowest of the Star Peak lime- 

 stones on the east side of the range, and is observed above Buena Vista and 

 Cottonwood Canons to abut against the granite high up near the head of 

 Wright's Canon. 



Where the limestones and underlying quartzites are wrapped around 

 the northern boundary of the gneisses, they bend to the east and acquire a 

 strike of north 40° east and a dip of 50° to the northwest. Here at the 

 contact between the quartzites and limestones there is a transition series of 

 15 or 20 feet thick, formed, at quite regular intervals of 4 inches, of inch- 

 thick beds of quartzite and limestone. About one-half mile north of the 

 northern termination of the Archaean schists, the limestones are observed to 

 have a strike of north 35° east, and a dip of 38° to the southeast into the 

 range, but this appears to be very local and due to faulting. The main 

 limestone and quartzite formation from here north has a strike of about 



