RANsome.] OUTLINE OF GEOLOGY. 85 
series of old quartzites and slates has been assigned to the Algonkian 
period, chiefly from their lack of fossils, and because they may be 
thus plausibly correlated with the Algonkian rocks of the Grand 
Canyon of the Colorado. 
The earliest Paleozoic formation is a quartzite with some sandy 
shales 100 to 200 feet thick, which is seen on the west side of the 
Animas from the monzonite contact to Molas Lake, and imperfectly 
on the east side of the Animas. This quartzite has been traced down 
the Animas to a point below Rockwood, and is called the Ignacio 
quartzite from its characteristic development on the bench where the 
lake of that name is situated. A southerly dip carries this quartzite 
onto the southern slope of the Needle Mountains, and there a few 
indistinct fossils have been found which indicate its Cambrian age. 
The rather shaly beds, often calcareous, succeeding the quartzite 
have as yet yielded no fossils. If there are any Silurian strata in 
this section they are probably represented by these calcareous shales 
and sandstones. 
The nexl formation recognized is a heavy white limestone about 
200 feet in thickness, which is seen resting directly on the Algonkian 
suartzites jusl south of Ouray, and has been called from this locality 
the Ouray limestone. In the Animas region this limestone rests upon 
the calcareous shales jusl mentioned. A few Devonian fossils were 
found in this Limestone by F. M. Endlich during the Hayden survey. 
At the present time a quite characteristic Devonian fauna has been 
obtained from this limestone al various places. 
Following the Ouray limestone there is a strat igraphic break. 
Some formation of Lower Carboniferous age should appear at this 
horizon, and some fossil evidence lias been obtained to show that at 
least remnants of that formation may be found on careful search. 
But the great series of Upper Carboniferous rocks called the Her- 
tnosa formation come nearly or quite in contact with the Ouray lime- 
stone in many places. This series of alternating limestones, grits, 
ind sandstones is about 2,000 feet thick and forms the imposing 
scarp facing the Animas Valley on the west side. The same beds 
'orm notable cliffs about the town of Ouray. From the limestone 
nembeis a very characteristic Upper Carboniferous fauna has been 
)btained. 
At Rico the uppermost division of the Carboniferous was found to 
)e characterized by invertebrate fossils of the Permo-Carboniferous. 
Phree hundred feet of strata have been grouped together as the Rico 
ormation. Evidence is not yet sufficient to demonstrate the presence 
>f the Rico strata in the Silverton quadrangle. 
Following the Carboniferous comes the well-known series of reddish 
rits, sandstones, and conglomerates ordinarily known as the Red 
teds. This series has been termed the Dolores formation from its 
ccurrence along the Dolores River, and evidence of its Triassic age 
