EC. LA 9 RA A 
OBSERVATIONS ON THE GEOLOGY OF THE ROUTE. 
FROM THE COLORADO CHIQUITO TO SAN PEDRO. 
FROM THE COLORADO CHIQUITO TO THE GREAT COLORADO.—MAGNESIAN LIMESTONE.—CAÑON DIABLO IN LIMESTONE.— SIMILAR LIME- 
STONE AT OTHER PLACES.—GRANITE, OLDER THAN THE MAGNESIAN LIMESTONE.—EXTINCT VOLCANO, CALLED SAN FRANCISCO 
MOUNTAIN.—LAVA STREAMS.—SECONDARY CONES.— VOLCANIO ASHES.— DIRECTION AND EXTENT OF THE LAVA STREAMS. —SAND- 
STONE.— Bizz WILLIAMS’ MOUNTAIN, VOLCANIC.—ITS LAVA STREAMS.—CARBONIFEROUS LIMESTONE.—PICACHO, OF GRANITE.—CAn- 
BONIFEROUS LIMESTONE AND FOSSILS, —ÁZTEC MOUNTAINS.—HORIZONTAL STRATA FORMING A CLIFF.—LAVA.—-CYGNUS MOUNTAIN. — 
AQUARIUS MOUNTAINS.—GNEISS AND METAMORPHIC ROCKS.— TERTIARY STRATA.— TRAP DYKES.— VALLEY OF THE HAWILHAMOOK.-— 
CERBAT MOUNTAINS.—VOLCANIC ROCKS.— COLORADO RIVER.— COLORADO RIVER TO SAN PEDRO.—GRANITIO RANGES.— GREAT 
THE SLOPES.— CAJON Pass.—SaNDSTONE, PROBABLY TERTIARY.—GRANITE AND METAMORPHIC ROCKS.— lERTIARY sTRATA.— 
Fossizs.— RECENT ELEVATION.—BITUMEN AND TAR SPRINGS.—TRAP-ROCK.—ALLUVIAL. 
I. COLORADO CHIQUITO TO THE GREAT COLORADO. 
Magnesian limestone.— After crossing the Colorado Chiquito and ascending the western bank 
as far as Camp 86, Mr. Marcou found sub-schistose red sandstone with ripple-marks, and red 
clay. "These were underlaid by a magnesian limestone, or thick dolomite, stratified regularly in 
beds six inches to one foot in thickness. These strata were conformable with the red sandstones, 
and, like them, were inclined towards the north, at an angle of ten to fifteen degrees. Some 
of these limestone beds alternate with the sandstone, and were fossiliferous, but the fossils were 
very badly preserved. (Notes, December 22.) "This limestone was found along the trail con- 
tinuously for four miles, when it was hid from sight by sheets of lava. A specimen of lime- 
stone from the Colorado Chiquito (No. 62) is probably from this locality. A description of it 
will be found in Chapter X. 
This limestone forms a broad mesa or table-land just south of Camps 86 and 87, and this was 
traversed by Mr. Campbell and Captain Whipple. The former describes the limestone as 
exactly like that afterwards found further west, and which contained fossils characteristic ot 
the Carboniferous period. The stream or valley called Caron Diablo is in this rock. The beds 
dip gently towards the north, and thus do not form any bluff towards the river ; but the borders 
of the Cañon Diablo are precipitous, and the bluffs in some places are one hundred feet high, 
and entirely of limestone. Captain Whipple describes this cañon, in his report, as a chasm one 
hundred feet deep, with sides so precipitous that it was impassable. The surface aroand was 
nearly level, and the cañon could not be seen until its very brink was reached. From here, 
westward, the surface of the country appeared nearly level. Similar limestone was seen at 
Camp 90, just at the southern base of the great volcano. Still further west, a similar limestone 
formed the sides of the ravine called Parke creek, north of Camp No. 94. These several out- 
crops of horizontal, or nearly horizontal, limestone strata, at very nearly the same level, indi- 
cate that they form one continuous bed; and this is rendered more probable by the similar 
mineral character of the rock at the different places. It is difficult to decide upon the geo- 
logical age of this limestone, which is not represented in the collection by fossils, unless we 
regard the outcrop found near Camp No. 96 as a portion of the same bed. This outcrop 1s also 
