Adams— Contribution to our knowledge of the Laurentian. 67 
is identical with many highly siliceous slates. No. VI is such 
a slate from the Engadine district in Switzerland and is as will 
be seen almost identical in composition with No. V. Siliceous 
bands from the Canadian slate quarries also have a similar 
composition. The alumina here is low on account of the pre- 
ponderance of quartz which also lowers the contents of alka- 
lies. The magnesia preponderates over the lime as before. 
No. VI lost 1°92 per cent on ignition before analysis and these 
figures do not therefore appear in the analysis as given above. 
Bir HH 1D VE Wie VI. VII. 
GNEISS. GNEISS. SLATE. SLATE. GNEISS. SLATE. GwNEISS. 
St. Jean de M. Trembling Lake. Wales. Melbourne. Rawdon. Tinzen. Rawdon. 
lh i a 61°96 57°66 60°59 64°20 74°70 79:97 54°89 
Qe 1°66 1°66 
ds 2 ge 19.73 22°83 19°70 16°80 8°88 8°62 13°67 
Fe.03 Rte 9°64 6°63 eS 
GO se... 4°60 774 7°83 4°23 
Les 4°33 4°43 
ict trace. trace. trace. “50 62 
2) 9 “30 1°16 Mg 2 ta 1:07 “16 5°63 
i 6 2 ee eS). 3°56 2°20 3°94 1°87 1°52 4°70 
iE 0 “19 60 2°20 3°07 vB) 64 io 
i 3) 2°50 Ear (ys 3.18 3°26 "95 2°30 8°34 
Ign 7a 1°82* 1°50 3°30 3°42 1°05 (2-764) 
99°55 100°77 100°03 99°65 99°08 100°44 100-00 
Total alkalies. 3°29 6°32 5°38 6°33 ese 2°94 10°29 
VIII. Ix. 
GNEISS. GRANITE. 
Trembling Mt., Carlingford. 
SO Ee eee sae es Rae: Ae 69°24 70°48 
Al,O3 Sr ear a ete ate as 14°85 14°24 
Fe.03 Se te es 2°62 a Ge 
DTA eect Sd, Jee eee "45 
GUO ea Napa ye Pt arG 1°48 
Ric) teh, Sea eo se Ot "40 
NEGO os ae a a Bee 4°30 3°66 
iO Se ee Pee ae Ae 4°26 
Ta ll ee 70 1:59 
99°56 Sosa 
Total alkalies. = 250s ne 8-63 1°92 
The fourth of these gneisses, No. VII, differs entirely from 
the others. The low content of alumina, combined with low 
silica, the high alkalies and the preponderance of lime over 
magnesia mark it off as quite distinct from the slates and 
gneisses just considered. If it be an altered sediment.it is one 
which has suffered very little leaching during deposition and 
must have been of the nature of a tufaceous deposit, or one 
formed from the rapid disintegration of an igneous rock hay- 
ing the composition of a basic trachyte or syenite. It is there- 
* Water. + Water and graphite (by difference). 
