42 HORN EXPEDITION — GENERAL GEOLOGY. 



2. Those about whose origin there is consideralile doubt, including a 



great part of the gneisses. 



3. Those that are clearly of eruptive origin. 



1. The only representatives of this division met with were the quartzites and 

 micaceous slates, whose exact stratigraphical position is i-ather doubtful. The 

 Pre-Cambrian at Gill's Pass, on the Hugh Rivei', is introduced by a thick mass 

 of quartzite, succeeded by micaceous schists and gneiss ; and in Stuart's Pass, too, 

 quartzites are intercalated as we approach the axis of elevation in the gneissoid 

 granite of Brinkley's Bluff. Representatives of this division were also found near 

 tlie head of EUery's Creek, forming an elevated ridge [vide Fig. 1/') on each side, of 

 which undoubted Pre-Cambrian rocks occur in the form of gneisses and schists, 

 accompanied by intrusive rocks. This ridge is composed of highly-altered glassy 

 quartzites and micaceous clay slates, apparently passing into mica schist, quartzites, 

 and clay slates, which dip at 85° N., their strike being W. 10° N. We have not 

 enough evidence to determine whether or not these rocks belong to the Pre- 

 Cambrian. Their relation to the gneisses on the south and north sides of the 

 ridge was not made out. It is therefore quite possible that they may belong to 

 the Ordovician System, and form a portion of a northern extension of that system, 

 which had been faulted down so as to present the appearance of forming part of 

 the Pre-Cambrian System. On the other hand, it is also possible that they may 

 represent Pre-Cambrian sedimentary strata, which have been intruded by granites 

 of Pre-Cambrian age, the latter being now represented by the gneissic granite to 

 be found on the south of this ridge, and by the gneiss, etc., occurring on both 

 sides of it. 



2. Numerous varieties of gneiss occur within the Pre-Cambrian area, some of 

 which will be briefly described in a separate paper dealing with the microscopic 

 characters of these rocks. 



Specimen No. 29 represents a very typical variety of gneiss occurring in the 

 McDonnell Ranges. It is strongly foliated, but the folia, which usually have a 

 thickness of from quarter to half inch, are not very persistent. They pinch out, 

 occasionally very rapidly forming lenticular masses, which usually in this variety 

 are much drawn out. These lenticular masses are sometimes, however, very short 

 and " eye"-like. The folia, which consist for the most part of quartz, felspar (a 

 great portion of which is a plagioclase with low angles of extinction), and a few 

 grains of biotite, are separated from one another by thin black layers of biotite not 

 more than one-eighth inch thic'<. These thin layers of biotite are usually roughly 

 parallel to one another, but occasionally they intersect. 



