204 
IDDINGS. 
Table VII. 
Silica Percentages of the Poclcs from Electric Peak. 
53-72 
58.10 
6I.5O 
65.80 
55-23 
58.11 
61.85 
65-94 
55-64 
58-49 
63.OI 
65-97 
56.28 
58.87 
63.78 
66.05 
56-33 
59-64 
64.85 
67-54 
57 - 12 
60.54 
65.II 
69.24 
57-38 
60.56 
65.48 
58-05 
60.89 
65.60 
They were intended to supplement the complete analyses 
in order to demonstrate what is evident from the microsco¬ 
pical study of the thin sections, namely: that the different 
varieties of porphyrites and diorites pass through all possible 
gradations from one extreme to the other. The character of 
this transition is shown by the accompanying diagram, in 
which each determination is given the same weight, the 
series being arranged according to the increase of silica, and 
the silica percentages plotted as ordinates. 
The next three analyses of Table VI, Nos. 3008, 2724, 2695, 
are from the main body of the diorite, No. 3008 being a light 
colored vein or d?ke of diorite in the darker mass. Nos. 
2668 and 2676 are diorites from the main stock ; and Nos. 
3001 and 2670 are from the quartz-mica-diorite-porphyrite, 
occurring within the main stock. These latter analyses 
vary from 64.85 to 69.24 per cent, of silica. 
The variations of the other chemical constituents are best 
