58 Tlic American Geologist. •^"'^■' ■^^*'-- 
a succession of short sharp folds of extreme complexity- The 
gneisses at this point are bordered on the east by a large area 
of massive granite, and the disturbance doul)tless took place 
during its intrusion. The sharp folding continues for a short 
distance west of the granite, beyond which the gneisses grad- 
ually resume their normal attitude. Similar local distur1)ances 
due usually to granitic or other intrusions occur also at 
several points in the Yukon valley and elsewhere, but are 
always limited in extent, and taken altogether affect only a 
small proportion of the total gneissic area. 
The gneisses in some of the larger areas have been 
thrown into wide regular folds, in others the dip is constant 
in the same direction, the clastic schists underlying them at 
one boundary and overlying them at the other. This occurs 
both in the case of narrow dyke-like occurrences and wide 
areas several miles across. 
The strikes of the gneisses in the Yukon vallely and ad- 
jacent region are generally northwesterly. Northeasterly 
strikes were observed in a few places but are exceptional, and 
occasionally the planes of schistosity follow curved courses. 
The strike of the gneisses is often conformable to that of 
the clastic schists but not universally so. 
Jointage planes are conspicuous in the more massive var- 
ieties of the gneiss, but are so exceedingly irregular both in 
strike and dip that it was found impossible to make out any 
relationship between them. The joints cut the schistosity at 
all angles, and no distinct sets showing even approximate 
parallelism in strike were determined. 
Previous Vicivs. 
The gneisses briefly described above were examined by 
Dr. Willard Hayes in 1891 in the course of a hurried recon- 
naissance from the Yukon to Copper river, and referred by 
him altliough with some hesitation to the Archean.* n 1896 
the occurrences in the Fortymile district were examined in 
some detail by Mr. J. E. Spurr. Mr. Spurr after discussing 
the time relationship of the gneisses to the other rocks in the 
district states thatt "It may therefore be concluded that the 
Fortvmile eranite is older than the schistose sedimentaries 
♦Expedition through the Yukon District, >lat. Geog. Mag., Vol. IV. 
^Eighteenth .Annual Report. U. S. Geo. Survey, Part III, Page 137. 
