ENPLicH.] ACIDIC VOLCANIC ERUPTIVES TRACHYTE. 215 



rocks and strata by plutonic or volcanic seismic action. Lava is in- 

 jected into these fissures presenting, eventually, the most typical ap- 

 pearance. Belonging to this group are the — 



Intrusive eruptions. These latter are distinguished from the former 

 by their relative position to the rocks adjoining. An anarhactic erup- 

 tive will usually be so located as to stand at approximately right angles 

 to the stratification or structural planes of the formations it traverses. 

 In case it becomes intrusive, however, which only can occur in sedimen- 

 tary or in stratified beds, the foreign material will appear as one of the 

 members, will be in part conformable with them, and partake of their 

 various inclinations and deviations. 



Scattering eruptives comprehend groups of small, isolated volcanics. 

 They are intimately related as regards genesis and age, but appear as 

 studding, mostly, one of the younger sedimentary formations. 



These divisions will not be found to apply to all the sub-groups of 

 trachytic rocks. Some of them show one or the other feature only. 

 The subjoined groups of the entire trachyte series are based mainly 

 upon the observations made in the San Juan Mountains. I first pro- 

 posed it two years ago* and have since found the classification corrob- 

 orated wherever I have met with the group. In case we have before 

 us an anarhactic occurrence, it becomes a matter of difliculty to decide 

 to which subgroup of trachyte it should be referred, unless it is in direct 

 connection. Lithological characteristics alone will not always suf- 

 fice to admit of final discrimination. It seems highly probable that 

 many of the dikes and dike-systems we observe, though but a short 

 distance removed from the trachorheites would most probably correctly 

 be regarded as much younger. 



Trachyte No. 1 .t — The rocks belonging to this group may be regarded as 

 accessories. In the southern edge of South Park they were first noticed, 

 in connection with the andesites.f I have characterized the occur- 

 rence there as andesitic tuff. But tew localities were found before 1874 

 which furnished satisfactory opportunities for comparison and study of 

 this group. As soon as they were found, however, it became apparent 

 that the tuff belonged-, most i>roperly, to the trachytic series. I am in- 

 clined to the opinion that the tuffs were ejected after the cessation of 

 andesitic eruptions, and immediately preceded those of the trachytes. 

 In its generic character the trachyte No. 1 forms a peculiar group. Al- 

 though not fully answering the requirement of what has been de- 

 scribed as an isolated group, it possesses many of them. 



Essentially the members of this group are composed of what the 

 Italian calls lapilli, of finely separated volcanic ash and fragmentary 

 portions. Almost every volcanic eruption of the present day is either 

 preceded or lollowed by an ejection of " ashy" material from the crater. 

 The fate of Pompeii and Herculaneum show to what an enormous ex- 

 tent these ash-eruptions can increase even within the historical period. 

 I regard, therefore, the ash accumulations observed in the same hori- 

 zon, at so many localities, as the beginning of the trachytic series. 



Inasmuch as the ejections of ash and tuff takes place sporadically, 

 and as the material has not that nature which allows it to spread over 

 any considerable area, it will be found that the trachyte No. 1 is found 

 only in isolated patches. Its horizon in the vertical scale is well estab- 

 lished and constant, but its horizontal distribution is more or less acci- 

 dental. A number of typical localities were observed both in 1873 and 



* Ball. U. S. Geol. Surv., No. 3, second series, May 15, 1875, p. 151. 

 t Rep. U. S. Geol. Surv., 1874, p. 195. 

 t Rep. U. S. Geol. Surv., 1873, p. 320. 



