[DRESSER] THE BASIN OF LAKE ST. JOHN, QUEBEC 127 
Roberval. The name Roberval is proposed for the granite and 
gneiss which constitute the next younger formation in this district. 
While there is objection to the introduction of local term in a list of 
formations as is given above especially in such a way as to make it 
equivalent in importance to Laurentian, it is a manifest convenience 
in description to have a name. Also in the district under discussion 
the formation designated as Roberval, occupies a larger area than any 
other. It comprises granite, granite-gnesis and pegmatite, which con- 
sist essentially of microcline with subordinate amounts of oligoclase 
and labradorite, quartz, biotite and hornblende. Pyrite, magnetite, 
sphene and apatite are common accessories. Portions of the formation 
are massive, but the greater part is gneissic in structure. The large 
crystals of microcline in many places give rise to an augen-gneiss. 
Petrographically this formation is not essentially different from acid 
phases of the Laurentian, but it is distinguished from that formation 
by its structural relations. 
The Roberval is plainly intrusive in all three of the earlier for- 
mations. Evidences of such relations may be found in lot 1, range 1 
of the township of Metabetchouan and in that immediate vicinity. 
Here crystalline limestones and anorthosites are cut by numerous 
dykes and arms of Roberval granite, and in lot 2 of the same range 
Laurentian is intruded by anorthosite. Roberval and Laurentian 
were not found in contact at this point, but in the III range of the 
township of Ouiatchouan lot 9, near the road leading from Roberval 
to St. Hedwidge, and in other places in the same range, stocks of 
Roberval granite pierce Laurentian gneiss, where they send off 
dykes and apophyses and deflect the strike of the gneiss in a manner 
characteristic of intrusive contacts. 
The Roberval has not been hitherto separated from the Lauren- 
tian, but intrusions of granite in anorthosite have been noted by 
Dr. Adams and Dr. G. A. Voung.! Also Dr. A. E. Barlow? dis- 
tinguished a similar rock formation in the Chibougamau region which 
he designated as massive granite, noting that it was younger than the 
anorthosite of that district. 
The later limit of its age cannot be actually determined more 
precisely than as pre-Trenton. The Roberval, however, was foliated 
and eroded before the deposition of the Trenton, and is therefore 
assumed to be of Pre-Cambrian age. A granite apparently quite 
similar in character and in age relations has been described from the 
1Summary Report, Geological Survey, 1900. 
2 Barlow, Dr. A. E., ‘Report on the Chibougamau Mining Region,” Dept. of 
Mines, Quebec, 1910. 
