VOLCANOES AND LAVA-VALLEY OF THE RIO GRANDE. 
33 
axes are perpendicular to the cooling surfaces.” 1 We see from this description that the dyke 
was injected into the soft stratified rocks, which have since crumbled away and have left the 
harder igneous rock standing like a wall. This is often seen in a region where trap-dykes have 
traversed softer strata. A fine example is cited hy Owen, and figures are given of trap-dykes 
standing out from the surrounding surface, and exhibiting the columnar structure so perfectly 
that at a distance they appear like great piles of wood. I have often heard of the u stone walls” 
of this part of New Mexico, and have found it difficult to convince some that they were not in 
reality works of art. 
There are two or three specimens of the rock of these dykes in the collection, (see Nos. 31, 38, 
and 39,) and they are described in Chapter X. No. 38 is said to form a dyke in the white chalk, 
and is from the Galisteo river. Whether it is taken from the dyke mentioned in the notes as 
at Galisteo, is not evident. The rock, however, closely resembles the hard dark-colored trap 
of the Palisades and Connecticut. The rock of the dyke between Tuerto and Galisteo is very 
different ; it has a grey color, and consists of feldspar and crystals of hornblende. I find 
in the collection (No. 42) a specimen labelled c< Porphyre en dyke” from the volcano of Cerrito , 
near Santa Fe. This dyke is not mentioned in the notes. 
In addition to these records of igneous action, we find that the granite and metamorphic rocks 
of Gold mountain are traversed hy numerous veins of quartz, and these are also mentioned hy 
Mr. Marcou as traversing the granite of Sandia mountain, not far west of its junction with the 
limestone. From all these facts we may conclude that the eastern flank of all these ranges has 
been subjected to violent igneous disturbances subsequent to the uplift of the Carboniferous 
limestone, and even as late as the white sandstone strata of the Llano. 
Volcanoes and lava .—The first volcanic phenomena observed along the line of the survey was 
at Sieneguilla or Cerrito, a rancho on the Santa Fe river about fifteen miles southwesterly from 
Santa Fe. At this point Mr. Marcou found several small extinct volcanic cones called Cerritos , 
and overflows of lava which capped the strata along the stream. Specimens of these lavas are 
in the collection, Nos. 43 and 44, and descriptions of them will he found in Chapter X. We 
have no knowledge of the elevation or magnitude of these volcanic cones. 
The presence of a horizontal plain of basaltic lava on the western side of the river hack of San 
Felipe has been known for a long time, and a plate showing the bluff hack of the town is given 
in the report of Major Emory, and he remarks the presence of basalt crowning the table-lands of 
the west side of the river. 2 He afterwards observes that these basaltic coverings are characteristic 
of the valley of New Mexico. From Mr. Marcou’s notes I judge that the Rio Grande at this point 
cuts through the field of basalt, it being found on both sides of the stream. He observes, 
October 4th : “ We see this black lava spread out, and forming the summit of the cliffs on the 
other side of the river. A broad sheet of basaltic lava was also found spread out over the 
surface of the table-land between the Rio Grande and the Puerco, and along the route taken hy 
the survey between Camps 61 and 62. This is over thirty miles south of the lava at San Felipe, 
and is directly west of Isleta.” 
Sedimentary deposites of the Valley of the Rio Grande .—The table-lands of the western border 
of the Rio Grande attain an elevation above the streams in some places of about 500 feet, and 
present a bluff, precipitous margin towards the east. Little is yet recorded of the lithological 
character of the strata composing these table-lands, or mesas , as they are there called. The 
hold bluffs certainly present a fine opportunity to the geologist for obtaining sections. 
It is believed that these bluffs reveal very nearly the same composition and arrangement of 
strata as the bluffs of the Llano. 
Mr. Marcou notes the occurrence of Cretaceous strata opposite Alameda. They are sandy and, 
decomposing, and hy wearing away expose a layer of red marl below. 
1 J. W. Abert’s Report of an Examination of New Mexico, p. 452. 
2 Emory’s Report, p. 39. 
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