ENDLicH.] GEOLOGY— SAN LUIS DISTEICT, SECTION B. 325 



syenite, as it resembles it quite closely in its lithological character. The 

 color is gray, the texture fine to middle-grained, and very uniform. The 

 feldspar is light gray, white, yellowish, and small crystals of white 

 oligoclase are dispersed throughout the mass. Quartz is colorless to 

 white, the mica black, sometimes in crystals. The position that this 

 granite occupies with reference to the superincumbent sedimentary 

 strata, renders it of the highest interest. In the resume of section fo, 

 this will be spoken of at more length. Over to the northwest, toward 

 station 1, the granite assumes a slightly gneissic character, owing to a 

 considerable quantity of mica contained therein. 



. Gneiss occurs at one point in section &, but has changed its character 

 to a considerable degree. Hunt's Peak, at the northwest end of the 

 Sangre de Christo range, is composed of gneiss, and from there it ex- 

 tends for about six or seven miles to the northwest. A thorough meta- 

 morphosis has taken place, and the micaceous parts have agglomerated 

 in such a manner as to appear like concretions, and are contained 

 between the planes of feldspar. In weathering, the quartz, feldspar, and 

 smaller mica particles fall oif, leaving these button-shaped nodules pro- 

 truding on the faces of the rocks, thus presenting a very peculiar ap- 

 pearance. Besides the white micaceous portions, black mica is dissemi- 

 nated throughout the rock. The quartz is white, in small particles, the 

 feldspar in very small quantities, besides the large buttons, mica, both 

 black and white predominating throughout. Although having stratified 

 structure, the rocks, thanks to metamorphosis, weather more like gran- 

 ite. 



Hornblende-rock may be the general name for a group comprising sev- 

 eral distinct species of rocks. It has been a question how the horn- 

 blendes of the Eocky Mountains are to be considered, because minera- 

 logical differences frequently cannot be found between specimens from 

 narrow local dikes and specimens of a rock that builds up mountains. 

 All that are found in section h are merely crystalline aggregates, with- 

 out any distinct i)aste, therelore must be excluded from the category of 

 volcanic eruptive rocks. In the eastern portion of the section, horii- 

 blendic material occurs only in the form of dikes, and will be treated of 

 under that head ; but at station 15, toward the west, we find these rocks 

 in more considerable masses ; several hills of 500 or 600 feet relative ele- 

 vation are composed of it. The structure is that of -a stratified rock, not 

 infrequently inter-stratified with bands of granite, mostly having a gen- 

 tle dip, in this instance to the eastward. It is composed of small crys- 

 talline particles of hornblende and oligoclase, joined together rather 

 loosely, without the slightest intimation of a magma. Small rounded 

 nodules of a green mineral that seems to be pargasite are dispersed 

 throughout it. Although, mineralogically, there would be no objection 

 to calling this rock diorite, lithological features do not correspond. This 

 point is perfectly isolated, located near the beginning of a long, narrow 

 patch of trachorheite , at first suggesting its being propylite. On the 

 western slope of the Sangre de Christo range more horublendic rocks 

 are found. Immediately west of station 16 they begin, forming a nar- 

 row belt near the base of the mountains, and there they are dioritic. 

 Hard and tough, they resist alike the influence of atmospheric condi- 

 tions and the geologist's hammer, while the scaly plate-like fragments 

 produce an uncomfortable ascent. Farther north in the range, near 

 Hunt's Peak, another mass of these rocks occurs, forming some of the 

 high peaks of that neighborhood. Again they differ in character, how- 

 ever, and instead of a diorite we have hornblende-schist. Thin nar- 

 row crystals of hornblende, in immense quantities, are mixed with a 



