348 GEOLOGICAL SUEVEY OF THE TEERITOEIES. 



its peculiar constitution to the close proximity with volcanic rocks ; and 

 lastly, the eruptive granite of station 38. Throughout, as well in lithologi- 

 cal and mineralogical character as in the structure of mountains, the three 

 main varieties can readily be distinguished. In other countries granites 

 have been found, that are said to reach back only to very late geological 

 ages ; here we have one the protrusion of which I think falls into a 

 Post- Carboniferous period. The evidence afforded by the Sangre de 

 Dhristo range of section h is corroborated by the observations made 

 here. Less noticeable than in the two preceding sections is a gradual 

 or sudden rise of the main granitic mass, which, if it has occurred, 

 must have been but slight, and not of long duration or great power. 



An accumulation of hornblendic rocks is a curious and interesting 

 feature, and the similarity they bear to alpine and other euphotides, 

 combined with the fact of their usually reaching very high altitudes, 

 may point to an analogy of formation and relation to surrounding rocks. 



Astonishing is the mass of volcanic material that had accumulated 

 and found its points of outflow in this section, at the same time show- 

 ing no very massive deposits at aify one point, as might have been ex- 

 pected. Upon the origin of this and the other volcanic effluvia, we will 

 speak in the '' Conclusion." 



COiS^CLIJSIOX. 



In concluding the report of the geognostic and geological features of 

 the district assigned to me for 1873, it may be of value to give, in gen- 

 eral outlines, the distribution of geological formations throughout it, 

 and to speak briefly of the correlation existing between them. 



In taking a bird's-eye view over the mountains of this district, it 

 becomes strikingly evident at a glance that two main systems of 

 mountain-ranges traverse the country : the one, geognostically granitic, 

 in a direction a little west of north and east of south ; the other, vol- 

 canic, crossing it at almost right angles, having a course of about 15"^ 

 south of west and north of east. 



Granite seems to be the oldest rock found there; but there are four 

 distinct varieties of it, three of which are considerably younger than the 

 first, the red to pink variety, mostly coarse-grained, abounding, with 

 local accumulations of the one or other constituent mineral. The second 

 type, the age of which will probably be correctly defined as Post-Devo- 

 nian, is that which occurs in the northeast portion of section c, form- 

 ing the high sharp j)eaks that are so characteristic to the range west of 

 the North Arkansas. While oligoclase is of exceedingly rare occur- 

 rence in the oldest granite, it is found to be the mineral second in quan- 

 tity only to orthoclase in this instance, imparting an entirely different 

 character both as regards appearance and weathering. Of probably the 

 same age we find the porphyritic x3rotogine of station 45, owing its con- 

 stitution most likely to some different circumstances experienced while 

 cooling. Eruptive granite is the fourth variety, occurring in the Sangre 

 de Christo range and on station 38, of Post-Carboniferous age, resem- 

 bling in its lithological character more closely that of Mount Princeton. 

 Although the term '' eruptive granite " is frequently used by European 

 writers, this has not been done to any considerable extent by Ameri- 

 cans ; and I would state that I do not consider it eruptive in the same 

 sense that basalt is eruptive, but merely wish to imply by that term 



