IGNEOUS ROCKS 203 



posits and accumulations. These include chiefly gravels, sands, clays, 

 muck, peat, soil, and ground-ice, which floor all the principal valleys of 

 the district and constitute a thin mantle overlying the consolidated rock 

 formations throughout the greater part of the district, particularly in the 

 less rugged localities. 



IGNEOUS ROCKS 



The igneous rocks of this belt are divisible into two main groups. The 

 one group is composed of somewhat basic intrusives, dominantly diabases, 

 diorites, andesites, and related types; the other group includes mainly 

 plutonic rocks of granitic habit and more acid composition, and are 

 mainly syenites, grano-diorites, and related rocks. 



The more acidic group of these rocks were noted in only three or four 

 localities and the exposures are all small. All these rocks cut the mem- 

 bers of the Orange group and are thus of Mesozoic or later age. All the 

 members of the more basic group that were examined proved to be dia- 

 bases, diorites, or andesites. The diabases occur mainly associated with 

 the members of the Tindir group and are in places extensively developed, 

 occurring as dikes, sills, and irregular intrusive masses in the Tindir 

 sediments. Occasional diabase dikes also pierce the overlying beds, and 

 some were noted cutting the Mesozoic sediments. Since the diabase is so 

 extensively developed in association with the Tindir members, however, 

 and but rarely cut the more recent rocks, it is apparent that the diabase 

 in this belt, although lithologically all very similar, ranges in age from 

 Lower Cambrian or pre-Cambrian to Mesozoic or later, but that it belongs 

 dominantly to the pre-Middle Cambrian. 



Summary and Conclusions 



A very complete and interesting section of the Paleozoic has been found 

 to occur along this portion of the Yukon-Alaska boundary, which adds 

 considerable, both stratigraphically and lithologically, to our knowledge 

 of this era in the northwestern portion of the continent. One of the most 

 important results of this work along the boundary is the finding of the 

 great thicknesses of limestones and dolomites which there occur. These 

 beds range in age from Carboniferous down to the Middle and possibly 

 also include the Lower Cambrian, showing that the deep sea reigned, 

 apparently continuously, over extensive portions of the region during this 

 tremendous period of time. Another very interesting conclusion concerns 

 the rapidity with which it has been shown that the lithology changes, and 

 consequently how uncertain and unsatisfactory lithological evidence has 

 proved to be, thus adding greatly to the difficulties connected with geo- 



