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Show 105 canic activity has produced a change in the mesa scenery, the basaltic Cerro de Cabezon resembling a gigantic sombrero^ and the Cerro de Alesna an imposing pyramid, forming a striking contrast with the trapezoid mesas in the vicinity. Basaltic protrusions in the Cretaceous formation of the Nacimiento Desert also occnr farther west, toward Fort Defiance. The Bandstone of the mesas is chiefly calcareous, and occurs occasionally in plates of 2 to 6 inches thick, separated from one another by " a thin crust of carbonate of lime. The sandstone plates were used by the former Indian inhabitants as building material for houses and fortifications, of whii- h a great number are found on the Canon de Chaco. A specimen of this sandstone was treated with hydrochloric acid, by which a complete conversion to the granules composing the rock Was effected, the cementing material, carbonate of lime, being dissolved. These granules consist chiefly of quartz, but feldspar, mica, and hornblende particles are readily discovered among them. The composition of the saudstoue is as follows: Granules... N '. 59.25 Clay > 2.23 Soluble in I Carbonate of lime 34.70 hydroohlo- < Carbonate of magnesia .. 2.43 ricacid. ( Oxide of iron 1.51 100.12 The amount of cementing material is here unusually large. An interesting fact is the occurrence of fine blood- red garnets in the western portions of the Nacimiento Desert. Over large areas, some ten or fifteen miles east ot Fort Defiance, we find these beautiful gems scattered in the loose sand. Their exterior surfaces are much worn, indicating, apparently, transportation from a great distance. Indeed, there is no rock in the neighborhood that might have originally contained them; but some fifty miles to the northward is a syenite, which contains large masses of garnets ; there may, however, formerly " have been garnetiferous syenite, or schists, in existence much nearer than this. It is worthy of mention that among the pebbles composing the ant- hills, it is usual to fiud the finest colors of garnets, which leads to the supposition that bright colors have a peculiar attraction for these ants. The finding of garnets in this region gave the first impulse to the memorable diamond- excitement of some years ago; and although that gigantic swindle is a thing of the past, these objects still bear the name " ruby- spiuel." Their hardness is 7, and specific gravity 3.75; while the average absolute weight is about one gram, and diameter ^ to i of an inch. On being analyzed the following was shown to be their composition : I. n. Silicic acid 45.80 41.35 Protoxide of manganese trace. 2.59 Lime 6.43 5.29 Magnesia 16.60 15.00 Perotoxideof iron 10.96 9.94 Proxide of iron 2.00 1 Alumina 19.25 22.35 Chromic oxide trace. 4.17 Total 101.04 100.69 For comparison the composition is given under II of a pyrope- garnqt from Bohemia, analyzed by Moberg, whicn shows that the blood- red garnets of Fort Defiance belong to this variety. There are, in fact, quite a number of varieties of garnets which differ widely in composition, but having a crystalline form closely agreeing with each other. The noble garnet, or almandine, is an alumina- iron garnet; the pyrope, an alumina- magnesia garnet; the grossularite, an alumina- lime garnet; besides these there are chrome garnets, lime chrome, lime- magnesia iron, and manganese- alumina garnets. They often occur of green, red, and violet color; also colorless and black. Together with the blood- red garnets of Fort Defiance are found green transparent grannies worn off like the former, so that their crystal faces cannot be recognized. At first I took them for green garnets; subsequent examination, however, proved that I was in error. Their specific gravity is 3.20; hardness, 6 ; and their chemical composition corresponds to that of the chrysolite, as follows: I. n. Silicic acid 43.02 43.44 Magnesia 48.15 49.31 Ferrous oxide 7.42 6.93 Alumina trace, trace. Lime trace, trace. Oxide of nickel 0.21 0.32 Oxide of cobalt trace, trace. 98.80 100.00 LL- 8 |
