THE GRAND CANYON OF THE COLORADO. 325 
side of the canyon the strata occupying the same relative position 
beneath the base of the Unkar terrane have been considered to 
be of Archean age. 
SEDIMENTS AND CONDITIONS OF DEPOSITION. 
Over the eroded, upturned beds of the Vishnu terrane a bed 
of siliceous conglomerate, composed largely of: pebbles derived 
from the beds below, indicates the old sea-beach formed during 
the period preceding the deepening of the water. In this sea 
sand and a few beds of calcareous mud accumulated prior to the 
spreading of a flow of basaltic lava which now forms a massive 
bed 80 feet in thickness. In the period of quiet following 
the lava-flow, a few alternating beds of calcareous and arena- 
ceous mud and sand were deposited prior to the deposition of 
5350 feet of sandy beds, which now form rather fine-grained, 
vermilion, chocolate, brown, buff, and parti-colored sandstones. 
With the close of this epoch of arenaceous deposition the sea- 
bed and the strata beneath were fissured by crevices which 
extended down into the Archean, and flow after flow of basaltic 
lava poured out through these over the sea-bed. In the intervals 
between the flows the deposition of the sand continued, and 
we now find, between the massive lava-flows evenly distributed 
beds of reddish-brown sandstone. With the last of the six prin- 
cipal flows the sea deepened, and a thick deposit of calcareous 
mud was accumulated, which now forms a magnesian limestone, 
passing below into a calciferous sandrock, the whole varying 
from 50 to 150 feet in thickness. This was the closing deposit 
of the Unkar terrane. Its upper surface shows slight traces of 
erosion, and, as the sediments of the succeeding Chuar terrane 
are unlike those of the Unkar, it is probable that the source from 
which the sediments were derived changed from one that: had 
contributed avast amount of sand to one that yielded great 
quantities of argillaceous matter and sand of a still finer char- 
acter. It is not to be understood that this implies a change of 
source of sediment, but rather a change of conditions, produced 
by progressive erosion that lowered a somewhat elevated area 
