ENDLicH.] ACIDIC VOLCANIC ERUPTIVES POEPH. TRACHYTE. 245 



Free silica lias not very often been observed in the porphyritic trachytes, 

 and, whenever found, the suspicion lay near at hand that it might have 

 been received during the passage of the lava through some highly 

 siliceous stratum. In consequence of their greater direct affinities to 

 trachyte, I have placed them so as to precede, in age, the rbyolito in the 

 first table, but have reversed this succession in the discussion, in order 

 to give exjiression to my doubts. Eowhere were rocks of this group 

 ibund in sufficiently intimate contact with the younger trachorheites to 

 admit of any definite interpretations of their relations. One feature 

 may be regarded as distinctive, however. While the trachorheites are 

 usually found to have their origin in regions occupied by metamorphic 

 rocks, the porphyritic trachytes are found in sedimentary areas. This 

 argues an inference that enters into consideration in the study of their 

 original sources. 



VII. — AGE. 



A certain amount of definite information has been collected with ref- 

 erence to the age of the porphyritic trachytes. It has been mentioned 

 above that they are found mainly in Cretaceous areas. At Park View 

 Mountain and south of Spanish Peaks dikes formed of it cut through 

 Post-Cretaceous strata (liguitic series). I am not aware that any strata, 

 younger than these have been traversed by them. As these occurrences 

 are thoroughly characteristic at both localities, and as their connection 

 with the entire group has been satisfactorily established, we may regard 

 the period of eruption as falling very near the beginning of the Tertiary 

 epoch. I am inclined to regard it as rather belonging to an era after the 

 beginning of Tertiary sedimentation. As each of these formations re- 

 quired enormously long periods of time for its deposition, we are not 

 enabled to make an assertion that would confine the eruptions within 

 narrow chronological limits. Large masses of Tertiary beds may have 

 elsewhere been deposited while the region exhibiting volcanic activity 

 had not yet been invaded by water or sediment. 



BASIC VOLCANIC EEUPTIVES. 

 DOLERITE. 



Among all the volcanic rocks in Colorado dolerite occupies the least 

 prominent position. Most likely many of the rocks that are classed 

 with the basalts should properly be referred to dolerites. It is a matter 

 of some difficulty to separate the two in the field, however, owing to 

 their mode of occurrence and to the similarity of lithological constitu- 

 tion. Perhaps the best criterion for separation of the two may be the 

 presence of olivine in the basalts, and this has been made the basis for 

 discrimination in the subjoined pages. Although, at first glance, the 

 occurrence of olivine might seem to afford a positive element of recog- 

 nition, the decomposition and eventual disappearance of this mineral 

 greatly increases the difficulty. Were it possible to employ microscopic 

 tests in every instance, but little doubt could remain as to the true 

 character of the rock. It is evident, however, that this means can be 

 employed in isolated cases only, and we are forced to base opinions upon 

 less reliable bases. 



Dolerite occurs in the Uncompahgre group in small quantities, associ- 

 ated with basalt. The connection of the two is so intimate, and the 

 former occurring in so subordinate a jjosition, that they could not 



