ALGAL PILLARS—-BROWN 
mineralization with the partial or 
complete filling of what had obviously 
been a cavity, that is, a space unfilled 
by mineral or organic matter for some 
time prior to the entrance of the solu- 
tions from which the chalcedony, 
quartz, and calcite were derived. 
The outer zone, sometimes 20 cm. 
thick, extends from the outer boundary 
of the chalcedony zone to the pe- 
riphery of the pillar. It consists 
of many thin, somewhat undulating 
laminae or layers of lime (CaCQ;) 
deposited one upon the other (pl. 4), 
thus causing “‘growth” of the zone in 
the direction away from the center, in 
contrast with the development of the 
chalcedony zone, which was toward 
the center of the pillar. The layers are 
fairly uniform in thickness and they 
are the result of a coalescence of 
materials laid upon closely spaced 
centers of origin in such fashion as to 
cause a succession of small, outwardly 
bulging arcs (pl. 3, fig. 2) around the 
pillar. All the layers with their mi- 
nute arcs and intervening depressions 
unite in giving this zone a somewhat 
spongy appearance and, when ex- 
amined with a hand lens, show that the 
zone is neither a cross section of fossil 
wood nor a section of a concretion 
with chemical rings or bands. Rath- 
er, it is quite evident that small, spher- 
ical algae, the molds of which are still 
visible between the layers (pl. 4), were 
involved in the deposition of this zone. 
The outer zone of these pillars is, 
therefore, identical in origin with the 
deposits comprising the reefs, algal 
pebbles, and coatings around fossil 
wood found in the shore phases of the 
Green River formation. Those in- 
crustations were attributed by Bradley 
(1929) chiefly to the activity of the 
alga Chlorellopsis coloniata. Johnson 
(1937) referred similar but smaller 
reef-building algae from the Oligo- 
cene lakes of South Park, Colo., to 
Oncobyrseila coloradensis. ‘The large, en- 
crusted logs described by Bradley 
(p. 210) simulate sections of the pillars 
from Whisky Basin, with the exception 
that silicified wood, instead of chal- 
817369 —49——_22 
279 
cedony, is the material of their inner 
zones. As these logs are usually found 
in prostrate position with much of the 
wood intact they have never presented 
any special difficulties as regards ex- 
planation of their origin. 
In many of the upright pillars from 
the Bridger Basin, the outer limy zone 
is more or less silicified, the silica 
filling the irregular pores and crevices 
in the spongy calcium carbonate or 
replacing it completely. It is clear, 
therefore, that this impregnation by 
silica and filling of the center by banded 
chalcedony and quartz occurred sub- 
sequently to the attainment of full 
outward size by the pillars, that 
is, after algal activity ceased. One 
section (pl. 6, fig. 2) bearing on this 
point is especially pertinent. After 
a considerable thickness of lime had 
been deposited, the pillar toppled 
over or was thrown over so that the 
side on which it fell was bashed in, as 
shown by the conspicuous ‘‘fault’’ 
line, angles, and pieces of broken algal 
segments within the enclosure. Fur- 
ther, the chalcedony ‘‘growth’’ lines, 
conforming to the angles and parallel- 
ing the outlines of the broken seg- 
ments, clearly indicate that the em- 
placement and banding of the silica 
was subsequent to the completion of 
the algal deposit. In some instances 
the innermost laminae of the algal 
zone appear to have suffered some 
solution or disintegration before silic- 
ification began. Here it should be 
stated that, although a general pattern 
of development can be described as 
applying to all the pillars, each 
specimen has individual peculiarities 
caused by variations in the environ- 
mental conditions under which it 
originated. By paying too much at- 
tention to these minor though inter- 
esting details and effects, it is quite 
possible to lose sight of the main 
process. 
Essentially, the chemistry of the 
process by which algae cause deposi- 
tion of lime from waters charged with 
that substance is regarded as simple, 
namely, that as these aquatic algae 
