GENERAL GEOLOGY 183 



tain of the Lower Cambrian or pre-Cambrian members, and occur as sills, 

 dikes, or larger irregular intrusive masses. The metamorphic rocks are 

 dominantly schistose in character, and within the area here being consid- 

 ered are mainly at least of sedimentary origin. These are developed only 

 along Yukon Eiver and are thought to be the oldest rocks in the district. 

 The metamorphic schistose rocks are all included under the term Yukon 

 group, the members of which within the belt here particularly under con- 

 sideration consist dominantly of schistose amphibolites, quartzite schists, 

 mica schists, and occasional beds of limestone. The evidence obtainable 

 concerning these rocks indicates rather conclusively that they are all of 

 pre-Cambrian age. The members of the Tindir grou.p may also prove to 

 be of pre-Cambrian age, and are at least older than Middle Cambrian. 

 This group of rocks is composed mainly of dolomites, quartzites, shales, 

 sandstones, and associated greenstones, which are considered to be almost 

 undoubtedly younger than the members of the Yukon group. The Tindir 

 group is overlain unconformably by a thick series of limestone and dolo- 

 mite beds which range in age from Cambrian to Carboniferous. In the 

 northern portion of the belt, beds of this character, all lithologically very 

 similar, overlie the Tindir members, and include beds ranging from Mid- 

 dle and possibly also Lower Cambrian age to Pennsylvanian, all the 

 Paleozoic systems being represented. Toward the south the upper mem- 

 bers are gradually eliminated, and the time interval becomes represented 

 by more arenaceous, argillaceous, and siliceous beds, including mainly 

 sandstones, conglomerates, shales, slates, and cherts, with some inter- 

 calated thin beds of limestone. Thus by the time Harrington Creek is 

 reached, at latitude 65° 05', the limestone-dolomite beds range upward 

 from Cambrian to and include Devonian members, which are overlain by 

 Devonian and Carboniferous shales, slates, cherts, and thinly bedded lime- 

 stones. About 10 miles farther south the shales, slates, cherts, and thinly 

 bedded limestones range in age from Carboniferous down to and include 

 Ordovician members which there overlie Ordovician and Cambrian lime- 

 stones and dolomites. Thus toward the south the argillaceous and arena- 

 ceous members are more persistent, and the limestone-dolomite members 

 represent a much shorter time interval than farther north. In the north- 

 ern part of the belt Mesozoic-Pennsylvanian sediments comprising the 

 Orange group, and consisting dominantly of shales, sandstones, con- 

 glomerates, slates, and quartzites, as well as occasional limestone beds 

 containing Pennsylvanian fossils, overlie the older terranes. This is the 

 most extensively developed formation along the 141st meridian between 

 Yukon and Porcupine rivers. Farther south the Mesozoic beds were not 

 recognized, but lithologically somewhat similar conglomerates, sandstones, 



