DESCRIPTION OF THE COLLECTION. 109 
mass or boulder. It is a compact, dark-colored trap, resembling the ordinary trap-rock of the 
Palisades along the Hudson river and East and West Rocks, near New Haven. 
No. 39. From between Tuerto and Galisteo.—According to Mr. Marcou, this rock forms a dyke. 
Its eruptive character is also clearly indicated by the specimen, it having the peculiar crystal- 
line grain and compact character common to recent intruded rocks, The mass has a pleasing 
grey color, produced by the intermingling of white semi-crystalline granules of feldspar with 
brilliant needle-shaped crystals of black hornblende. The rock may with propriety be called 
diorite, this name having been proposed for volcanic rocks in which hornblende is abundant, 
and mingled with feldspar. 
No. 40. Village of Sineguilla, near Santa Fé.—This specimen, like the preceding, is charac- 
terized by the presence of distinct crystals of black hornblende ; but it differs from the first in 
the nature of the base through which the hornblende is distributed. In this specimen it is 
compact and nearly homogeneous, being without the granular or crystalline character of No. 39. 
The color of the base is a bluish-grey, and the disseminated crystals of hornblende give the 
specimen the appearance and character of a porphyry. There is a general parallelism between 
the hornblende crystals of the rock. 
No. 41. Cañon de Tejeras, near Albuguerque.—This specimen much resembles a fragment of 
metamorphored slate, having a distinct lamellar structure, and cleaving more readily in one 
direction than in others. It consists of alternate layers of compact, dark-green argillaceous 
rock, with thin films of carbonate of lime. It is labelled Trap serpentineuse by Mr. Marcou ; 
but, although of a green color, does not closely resemble serpentine. The rock at a short dis- 
tance resembles ordinary trap or greenstone. 
No. 42. Volcano of Cerrito, near Santa Fé, in a dyke.—This specimen is compact, and repre- 
sents a uniform grey color, but is without any black hornblende crystals or other dark mineral. 
It has the appearance of a specimen that has been long exposed to the action of the atmosphere; 
and, although there are surfaces of fresh fracture, they are dotted here and there by brown 
stains, which, on examination, are found to be produced by the decomposition of a ferruginous 
mineral, the powder of which is strongly attracted and lifted by a magnet. "The mass is also 
full of small irregular cavities, some of which appear to have once been occupied by a mineral 
which has dissolved away. Crystals of white glassy feldspar are disseminated, and the rock 
may be called a feldspar porphyry. 
No. 43. Volcano of Cineguilla, near Santa Fé.—This is compact and fine-grained, but not very 
hard rock, of a faint lilac color, consisting of numerous disseminated imperfectly-formed crystals 
of white feldspar and distinct and well-formed crystals of hornblende. 
No. 44. Cañon of Sineguilla, near Santa Fé.—This specimen has the appearance and char- 
acter of a recent lava. It is hard, and like some trappean rocks in color; breaks with curved 
surfaces and sharp edges ; is amygdaloidal, having numerous air-cells ; somewhat elongated, 
or almond-shaped. These are hollow, and many are lined with a thin white crust of a zeolitic 
mineral, its surface being drusy with brilliant but microscopic crystals, so that the species cannot 
be determined. Other cavities are lined with a greenish-yellow crust, enveloping a siliceous 
nucleus. 
No. 45. From near the Pueblo de Zuñi.—This specimen is ticketed by Mr. Marcou as basaltic 
lava, from the end of the lava stream near the Pueblo of Zuñi. It resembles the preceding 
Specimen in its color and general characters, but is not so compact, being filled with small, 
irregular cavities, which render it more porous. It also contains large air-cells, or amygda- 
loidal cavities, but which are not lined with minerals. The mass is filled with small vitreous 
grains of a smoky tint, which are nearly or quite as hard as quartz. They may, however, be 
crumbled to a white powder under strong pressure by a hard steel-point ; and in this respect, 
and in a peculiar metallic lustre which they present in certain positions, they differ from that 
mineral, and leave me in doubt as to their true nature. They may be olivine; but the grains 
being so so small and obscure, it is impossible to examine them with satisfaction. 
