578 R. C. ALLEN AND L. P. BARRETT 



exception but the only thing known about this fault is its direction 

 and the fact that its vertical displacement is inconsiderable. In 

 reference to the structure of the west hill perhaps no inferences are 

 warranted. So far as known, the arkose of the lower series is not 

 exposed but the distribution of the lower and higher members of 

 the upper series together with the topographic expression faintly 

 suggests a shallow syncline trending across the hill in a N.E.-S.W. 

 direction carrying the quartz slate member in the trough and expos- 

 ing the underlying quartzite on its opposite flanks. But the 

 structure is probably not so simple as this for there is evidence of 

 faulting in some of the pits. 



THE LOWER (gWINN) SERIES 



Arkose and conglomerate. — The major portion of the arkose 

 formation is in reality now an abundantly sericitic quartzite, the 

 sericite being a metamorphic derivative of the original feldspar. 

 The abundance of sericite affords on cleavage surfaces, a charac- 

 teristic pearly luster. From the dominant phase there are grada- 

 tions through intermediate phases to typical arkose with feldspar 

 practically unaltered. Of subordinate importance are inter- 

 stratified lenses of conglomerate varying from a foot or two up 

 to eight feet in thickness. The pebbles are mainly vein quartz 

 well rounded and of various sizes under four inches in diameter. 

 Other pebbles of dense, vitreous, gray quartzite, black chert, and 

 siliceous dolomitic slate are much less abundant. The matrix 

 of the conglomerate beds has the composition of quartzite rather 

 than arkose and is usually dark, dense, vitreous, and slightly 

 sericitic. 



Bedding structure is not observable in any of the various phases 

 of the formation, except as it may be represented by an occasional 

 thin layer of gray chert. The deposition of these cherty layers 

 probably heralded the approach of a change in conditions of 

 sedimentation represented by an iron-bearing member in the 

 adjacent Gwinn synclinorium which lies in part directly on a similar 

 arkose-conglomerate formation. At Little Lake the iron-bearing 

 member appears to have been removed by erosion prior to the 

 deposition of the overlying conglomerate and quartzites. The 



