218 REPORT UNITED STATES GEOLOGICAL SURVEY. 



In tbat case rapid cooling would probably prev.ent its entering into 

 any composition which otherwise might have been effected. Thus, too, 

 does it seem easily explained why so often semiopal (quartz containing 

 water) should be found in such relative position. As will be seen far- 

 ther on, the question of hydration is a very important one in discussing 

 the origin and primary condition of volcauics. Therefore, any occur- 

 rence typical and frequently repeated, will add its share to the solution 

 of an otherwise comparatively intangible question. The obsidian, too, 

 of this locality shows special features of interest. The mineral is black 

 and contains small crystals of sanidite, thus becoming porphyritic. 

 Its structure is concentric at several places. Within the dike we find a 

 spherulitic, concentric arrangement of the obsidian. From small 

 spheroids they are found to the size of 10 feet in diameter. Similar 

 concretions have been noticed elsewhere in volcanic rocks. One of the 

 best known is the occurrence near Teplitz, in Bohemia. There a con- 

 centric arrangement is observed in the porphyries.* 



Trachyte No. 2,t — Less varied in its lithological features than the 

 former group, this one presents^a very uniform appearance. Compared 

 with JSTo. 1 and the succeeding No. 3, this group <loes not present such 

 marked characteristics as the other two. The rocks are by far harder 

 and more compact than those of the preceding group, and they are 

 better able to withstand erosive agents. Weathering iato compara- 

 tively small fragments of a light-brown color, this trachyte generally 

 presents gently rounded slopes near the base of bluffs or mouutainc. 

 Easily" decomposed, it furnishes soil, and an ample, unbroken veg<*- 

 tation not uutrequently denotes its presence. Exceptions from this 

 rule are not wanting, however. Trachyte No. 2 is found chiefly along 

 the main drainage of the San Juan Mountains. Taken as a whole, 

 the vertical as well as horizontal arrangement of the volcanics in that 

 region is extremely regular. We find, therefore, that in its greatest 

 depressions the lower beds and groups are mostly exposed. Great 

 uniformity of petrographic character will be found to exist in the 

 members of this group. As a rule the paste is microcrystalline, slightly 

 vesicular or compact. Numerous small crystals of sanidite are dis- 

 persed throughout, associated with diminutive crystals of black mica. 

 Prisms of hornblende, dark-green to black, occur si)aringly. . When 

 freshly broken the color of the rock is bright-pink, with a blueish tinge, 

 or reddish-brown. Upon exposure it soon becomes dull. Toward 

 the top of the group nodules and bauds of porphyritic pitch-stone and 

 obsidian set in. They are characteristic of this group, and almost en- 

 tirely confined to it. A total thickness of about 1,200 feet may be 

 accepted as the average for this group. On account of its toi)Ographical 

 position, I shall distinguish trachyte No. 2 as the " River group." Its 

 position along the borders of the main streams of the San Juan region 

 justifies the name. 



Although in no direct connection whatever, I am inclined to regard 

 the trachytes of the eastern portion of Colorado as belonging to this 

 group. Certainly their relative position to the andesites and tuffs 

 assigns to them this place. Lithologically, also, they agree very well 

 with the characteristics exhibited by the group elsewhere. It is natural 

 that an intervening space, devoid of volcanics, so large as the one in 

 this particular instance, should lead us to expect considerable changes 

 in the groups that must be regarded as belonging together. In its phy- 

 sical character, No. 2 belongs to '.hat type which has been designated 

 as massive eruptive. 



* Cotta, Geologische Bilder, Leipzig, 1871, p. 1. tRep. U. S. Ge.l. Surv., 1874, p. 196. 



