CAMBRIAN SEDIMENTS. 5 
derived from the alteration of the ferruginous minerals of the granite; it is 
either in the form of pyrite, in small crystals, or of specular iron or limo- 
nite, the former either in hexagonal crystals or in shells around an oxidizing 
erystal of pyrite, and the latter in irregular bunches disseminated through- 
out the rock. The successive alteration of pyrite to specular iron, or red 
hematite, and the hematite to yellow earthy limonite, in concentric shells, 
is often well shown; there is also occasionally some iron carbonate devel- 
oped as an alteration product. Besides this secondary iron there are occa- 
sional grains of magnetite and red hematite, which from their shape and 
position are evidently of detrital origin. 
Farther up in the beds the structure is essentially the same. With 
the diminution in size of the quartz grains the feldspar becomes more rare 
and the feldspathic cement almost disappears. The cementing material is 
then made up of secondary quartz, which has grown on to the original 
grains. ‘Tourmaline is also found among the detrital grains. 
Dolomitic quartzite —The upper third of this formation is not so compact and 
pure as the rest. In fresh specimens this rock appears pure and homo- 
geneous, but where it has been exposed to oxidation, as in all outcrops, it 
assumes a different appearance. The alteration is usually most marked 
along certain zones which are parallel to the bedding; along these the rock 
crumbles and is eroded, while the harder unaltered parts stand out, produ- 
cing a striking banding of brown and white. This tendency to oxidation 
increases toward the top of the series, so that in places the whole rock is 
altered. 
The microscope reveals very clearly the reason for this change. In 
the white, unaltered rock, there are found in the interstices between the 
rounded quartz grains, besides the secondary quartz, many scattered crystals 
of a carbonate, which, from its occurrence in isolated crystals of simple 
rhombic form and of grayish color, as well as by the analysis of the rock, 
is shown to be dolomite. This dolomite seems to have crystallized at 
the same time as the secondary quartz. That it was probably derived, 
however, from the alteration of original calcareous sediments is shown by 
the circumstance that it is much more common in certain zones than in 
others, and that these zones correspond to the bedding. In other sections 
it is shown that the dissolution of these dolomite crystals is the cause of 
the rapid alteration of the rock. By their removal, cavities are produced 
