32 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
it outcropping along the western side of the Sandia and Santa Fe mountains. As, however, 
it does not appear on that side of the mountains near the line explored by the survey, we must 
conclude that recent sedimentary deposites obscure it from view, or that it has been very much 
denuded so as not to appear. If we regard the granite as intrusive and as having flowed up 
in the manner of some trappean intrusions among stratified rocks, it is very probable that the 
limestone strata on the western side dip under it. The absence of the limestone may be 
entirely local, and it may be found to outcrop regularly and to dip west a few miles south of 
the line of exploration. 
Metamorphic rocks .—It appears from Mr. Marcou’s notes that this carboniferous limestone 
rests immediately upon the granitic and eruptive rocks, without any intervening sandstone or 
other strata. Neither does he record any evidences of a change or metamorphosis of the beds 
in consequence of their contact with these rocks. He, however, mentions, under date of Octo¬ 
ber 8th, that the green serpentinoid trap, which is the rock in contact with the limestone strata, 
contains strata of metamorphic limestone of Devonian age. The position of these rocks is 
shown by a figure. This serpentinoid trap appears in each of the small sections which Mr. 
Marcou has drawn in his note-book. 
With regard to the mineral characters of this trap, but little can be said beyond the descrip¬ 
tion which I have given of a specimen found in the collection. (See catalogue, No. 41, and the 
descriptions.) I have there remarked that the specimen greatly resembles a fragment of meta¬ 
morphosed slate. It has a distinct lamellar structure, and breaks more readily in one direction 
than in another. It is highly* argillaceous, and is interleaved with thin layers of carbonate of 
lime. It does not appear to contain any great amount of magnesia. These characters favor 
the belief that it is a metamorphosed calcareous and argillaceous shale. 
There is only one specimen of granite from these mountains in the collection, Nos. 1 and 2 
of the catalogue. The outcrop from which this specimen was probably taken is said to be 
three miles in width, and is succeeded by a rose-colored granite, with but little feldspar, and 
dykes of rosy-white quartz ; then follows the serpentinoid trap, with the Carboniferous strata 
resting upon it. There are no specimens from Gold mount or Santa Fe mountain. The drift 
or detritus coming from Gold mountain was noted as quartzose and containing crystals of 
pyroxene, or a similar mineral, about which there appears to have been some doubt. The 
detritus from the Santa Fe mountain consisted of granite, serpentine, &c. 
Trap-dykes .—Half way between Tuerto and Galisteo Mr. Marcou found a trap-dyke trending 
north and south. Further on, in the valley between the Old and New Placer mountains, 
several dykes were seen, one having a northwesterly and southeasterly direction, and they were 
all cutting the beds of the formation called “ Trias.” When at Galisteo another trap-dyke 
was noted ; it is ten feet wide, and traverses the white sandstone in a direction 30° E.E.N. 
to 30° W.W.S. The beds of the “Trias” were upheaved in the vicinity. (See notes, 
October 2.) 
I learn from A. H. Campbell, esq., the civil engineer of the survey, that there is a fine trap- 
dyke at the headwaters of the Bio Tuerto, near San Pedro Pass, which is forty feet thick, and 
trends S. 60° E. magnetic. The dip is 45° S. 20° W. It cuts directly through the red 
sandstones. The direction of this dyke is so entirely different from that observed by Mr. 
Marcou, that I cannot but regard it as separate and distinct. 
Lieutenant J. W. Abert observed trap-dykes on the banks of Galisteo river north of thePlacer, 
and describes one as follows : “Crossing the creek we entered a little vale, traversed in various 
directions by walls of trap. At one place we saw a wall that looked at a little distance as if 
made by human art; it was pierced as if for windows and doors. A stranger, whom we met, 
insisted that this dyke was one of the vestiges of the ‘ Indios,’ who lived here long ago. But 
the Cyclops alone could have worked with such vast materials as these. The planes of the 
joints and cleavage have formed the mass in fragments, consisting of rhomboidal prisms whose 
