GEOLOGY OF THE PARADOX LAKE QUADRANGLE 



479 



Sediments of the Grenville series. Probably the oldest rocks on 

 the quadrangle are metamorphosed sediments. Probably also only 

 one series of Precambric sediments is present. Six types of sedi- 

 mentary rocks are recognized : hornblende gneiss, limestone, mica 

 schist, silimanite gneiss, graphitic quartzite, shaly qnartzite. 



Hornblende gneiss. 1 Typically this rock is a very quartzose horn- 

 blendic gneiss. It is of gray color. Its sedimentary origin is indi- 

 cated by its large quartz content ; by certain persistent streaks of 

 brotite schist which appear to represent changes in composition 

 which could only be explained by changes in sedimentation from 

 sandy conditions to shaly ones; and by its vertical changes in min- 

 eral composition [see pi. 4, fig. 1]. The gneiss extends in a 

 horseshoe-shaped belt through the central, southern and western 

 portions of the quadrangle, containing within its area some of the 

 most important mountains, namely, Treadway, Putnam, Stevens, 

 Third Brother, Park and others. This gneiss is so excessively 

 crushed, and also so much altered by syenites, pegmatites and quartz 

 veins, that no structural features could be made out. 



Limestone. Closely associated with the gneiss occurs crystalline 

 limestone. It is a completely recrystallized rock, which presents 

 such remarkable metamorphic features that it was first supposed by 

 Emmons to be of igneous origin. Little trace of bedding is to be 

 found. While at times almost pure, it often contains metamorphic 

 minerals, such as graphite, apatite, pyroxene, amphibole, phlogopite, 

 biotite, scapolite, garnet, titanite, pyrrhotite and tourmaline. Most 

 of these minerals are clearly the result of regional metamorphism 

 acting upon impure limestone, but some of them, notably tourmaline, 

 titanite and scapolite, are the result of contact metamorphism. In 

 Moriah, just north of the region covered by this map, the limestones 

 are found charged with serpentine, forming the rock known as 

 ophicalcite. 



Gneiss is a laminated metamorphic rock having the mineral composition of 

 -a granite, but not necessarily in the same proportions. Varieties are indicated 

 by prefixing the name of the most important silicate, thus hornblende gneiss 

 is a rock containing quartz, feldspar and hornblende. 



