4 
MESOZOIC FORMATIONS. 
Cliama flows two miles west of the town, in a south by west course, through 
a bed cut in the dark lead-colored shales of Cretaceous No. 2. Eight miles 
northeast, an enormous vertical mass of rock rises abruptly 1,274 feet above 
the stream below its base, and is continued to the north and west in a less 
precipitous mountain-flank. This mass of rock is a land-mark over a great 
extent of country; it is cleft to the base by the canon of the Brazos Creek, 
one of the heads of the Chama. I took occasion on my return to traverse 
this upthrust, taking the trail which leads from Tierra Amarilla across the 
mountain-axis, of which it is the western border, to Conejos, on the edge of 
the Rio Grande Valley. 
The road follows the course of the Brazos River, and for some distance 
the Cretaceous beds are in sight and nearly horizontal. Near the precipice 
above mentioned, these are lifted into high hills at an angle of 70° and 80°. 
On the north side of the river, sandstones of No. 1 rise with a similar dip, 
forming the foot-hills of the mountain, which rises perpendicularly to 1,500 
feet. This mass is largely composed of a dense breccia of quartzite frag¬ 
ments, closely cemented into a uniform rock of a general pink color, and 
not variegated. Its characteristics and position refer it with probability to 
the Trias; but I could not detect any indication of the Jurassic beds 
between it and Cretaceous No. 1. After reaching the summit, we traversed 
the upturned edges of the formation, which have a strike varying from north¬ 
west and southeast to north and south. The elevated region now traversed 
by the trail is perhaps thirty miles in width, and is worn into rounded hills. 
The highest point indicated by the barometer is 10,400 feet. On the upper 
waters of the San Antonio Creek, high hills come into view, which have 
flat tops composed of a bed of trachyte, and their sides are often covered 
with pink and purple fragments of this rock. Within twenty miles of 
Conejos, the intervals between these hills are occupied by a heavy deposit 
of the Santa F(3 marls, which, with masses of intrusive basalt rising in irregu¬ 
lar masses, reminded us that we had once more reached the forbidding 
sceneiy of the Rio Grande Valley. 
Tlie bluffs that border the Chama near Tierra Amarilla are, as before 
observed, composed of the shales of No. 2, and they contain abundance of 
Ostrece and Inocerami. Near the upper part of the series, there are several 
