196 
IDDINGS. 
composed of ferromagnesian silicates and feldspar; parts of 
it are light-colored with a noticeable percentage of quartz. 
It is traversed by veins or dikes of light-colored diorites, 
and shows by brecciated portions that the whole mass is 
made up of a number of bodies that have followed one 
another up through the main fissure at short intervals of 
time. 
The minerals composing the diorites are: hypersthene, 
augite, hornblende, biotite, lime-soda-feldspar (labradorite), 
orthoclase and quartz. The relative proportions of these 
vary considerably as already noted. The crystallization 
of the coarser grained diorites appears to have been a con¬ 
tinuous, uninterrupted act; all of the minerals including 
the accessory ones having come within the influence of the 
causes producing the granular structure. In the finer 
grained, porphyritic portions of the diorite mass it is evident 
that the initial tendency of crystallization led to the forma¬ 
tion of hypersthene and augite, and also hornblende in 
some cases, and that the biotite was a later crystallization 
connected with the final consolidation of the rock. In the 
non-porphyritic, granular forms of the diorite) it is also 
evident that the earlier crystallizations were of hypersthene 
and augite and of part of the hornblende, while the greater 
part of the hornblende, and the biotite followed. The labra¬ 
dorite commenced to crystallize early, while the alkali 
plagioclase, orthoclase and quartz closed the series. 
The last eruption through this channel was that of a 
quite siliceous magma, solidifying as quartz-biotite-diorite- 
porphyrite, which approaches granite-porphyry in composi¬ 
tion. It is a light-colored, fine grained rock with abundant 
phenocrysts of plagioclase, biotite and quartz, with a little 
hornblende in places. It forms a broad body within the 
diorite, with a few dikes of still finer grained rock traversing 
the strata to the southwest. 
The mineral variation in the rocks within the stock is 
shown in the accompaning table in which (a) ( b ), etc., repre¬ 
sent different modifications of the rock. 
