‘SCUDDER. ] TERTIARY LAKE BASIN OF FLORISSANT. 271 
A.—THE FINER DEPOSIT JUST ABOVE THE SHALES. 
A medium-grained gray tufa, containing crystal and fragments of feldspar, augite, 
&c., cemented by a fine earthy groundmass. 
In the thin section it is seen to be an epitome of the volcanic rocks of the Cordilleras. 
The groundmass holds fragments of basalt, andesite, trachyte, and rhyolite, with de- 
tached minerals derived from them. 
The basaltic fragments have in part a dense globulitie base porphyritically hold- 
ing ledge-formed plagioclase crystals and a few augite granules. Some of the basalt 
is quite coarsely crystallized, approaching the doleritic type. Olivine was observedin 
some of the fragments, but it is largely altered to a reddish-brown serpentine. Mag- 
netite is abundant. In many of the fragments the groundmass has decomposed to a 
reddish-brown mass, which is untransparent and holds clear crystals of plagicoclase. 
The basaltic fragments have suffered more from alteration and decomposition than 
any others in the tufa. 
Of andesite, both varieties pointed out by us (1. ¢., p. 280) occur in this tufa. The 
first, which is nearest the basalt in composition, has a brown glass as its base, filled 
with microlites. This base holds minute rectangular and oblong crystals of feldspar. 
Large microlites of augite and grains of magnetite were seen. Fragments of this are 
common, and are clear and unaltered. The second variety of andesite was seen to 
have a dense gray micro-felsitic base, holding ledge-formed feldspars and magnetite 
grains. Some contained the reddish-brown fibers of the destroyed hornblende. Frag- 
ments of this variety of andesite are quite abundant. 
The trachyte has a light gray, felty, and glassy base, some fragments showing be- 
sides this only faint traces of polarization caused by incipient feldspars. Other frag- 
ments show minute, well-formed crystals that appear to be sanidin. Grains of mag- 
netite occur scattered through the base. This is also quite abundant, and it, as well 
as the basalt and andesite, surpasses the rhyolite in amount. 
The rhyolite occurs in the form of a more or less clear glass, often cellular. The 
cells are often drawn out in the direction of the original flow, forming a fibrous struct- 
ure, which when of a grayish or reddish brown color resembles woody fiber. Some of 
the fragments contain elliptical cells, and a few shards of water-clear glass free from 
inclusions were seen. 
Many crystals, entire or broken, are scattered throughout the groundmass of the 
tufa. These crystals belong to plagioclase, sanidin, olivine, magnetite, augite, and 
quartz. But little quartz was observed; one crystal contained trichites and vapor 
cavities. The trichites are the same as those commonly seen in the quartz of granite, 
but this appears to have been derived from the lava. The feldspar contains inclu- 
sions of base, glass, and microlites, and through these the rock from which the feld- 
spar was derived can often be told. The augites have the characters of andesitic 
augite. A little palagonite and one crystal of microline were seen. 
The groundmass of the tufa is composed of comminuted and decomposed material 
derived from the lavas before described. In the groundmass trachytic and rhyolitic 
material appears to predominate. This specimen was chosen for description, as it best 
represented the general characters of the tufas. 
B.—THE COARSER DEPOSIT JUST ABOVE THE SHALES. 
This is more coarsely fragmental than any of the others, and is composed of a yel- 
lowish brown earthy groundmass, holding fragments of quartz, feldspar, basalt, d&c. 
Some of the fragments appear to belong to the older rocks, but none of them were 
seen in the section. Under the microscope the tufais similar to the first one described, 
but its fragments are larger and sometimes better marked. Some kaolinized feldspars 
and a little biotite were seen. The hornblende in the andesite is in the usual broken 
forms, with blackened edges. 
C.—A SPECIMEN FROM FINER PORTION OF THE UPPER CONTORTED BEDS. 
A yellowish earthy groundmass holding crystals and fragments of augite and feld- 
spar. On one side is a layer of fine detritus, composed of the same material as the 
groundmass of the more coarsely fragmentai portion. Its microscopic characters are 
similar to those of A, except that its materials are more decomposed and sanidin is 
more abundant. One kaolinized feldspar was observed. 
D.—THREE SPECIMENS OF THE INSECT-SHALES. 
These are brownish and grayish brown shales, being simply the finer material of the 
tufas laid down in laminae of varying thickness and coarseness. One is very thinly 
bedded. 
This voleanic material has evidently been worked over by water, but the conditions 
can of course best be told in the field. So far, however, ag we can judge by micro- 
