238 EEPOKT UNITED STATES GEOLOGICAL SUEVEY. 



Dolores Peak and Lone Cone. — Mr. Holmes furnishes a section extend- 

 ing from Mount Wilson to Dolores Peak and thence to Lone Cone.* 



From this it would appear that both are essentially but a continua- 

 tion of the Mount Wilson upheaval. The trachytic mass has penetrated 

 the older mesozoic beds, has broken through the massive Dakota sand- 

 stones, and finds an opportunity for lateral expansion in the Middle Cre- 

 taceous shales. Dikes and intrusive sheets of volcanic material are 

 not wanting. The latter appear either as interbedded or in the form of 

 wedges. 



Viewed as a whole, the three eruptions undoubtedly belong together. 

 They occur in approximately a straight line, and their general as well 

 as detail character is perfectly conformable. It is a significant fact 

 that mainly the Middle Cretaceous shales are made the recipients of 

 lateral intrusion. Evidently the lower sandstones did not offer the same 

 facilities. Whether this may be the result of pressure produced by the 

 superincumbent strata, or whether the uplifting force was not sufficient 

 to produce a parting of the beds, is a question for investigation. It 

 seems a very difficult matter to measure the relative power of the force 

 employed, but phenomena of this character may eventually furnish some 

 tangible clue. 



Bear River Mountains. — A portion of the Bear Eiver Mountains may 

 be said to belong to the porphyritic trachyte group. During 1874 I vis- 

 ited the eastern part and found rocks that must be referred thereto.t 

 A mountain, upon which Station 36 was made, rises to an altitude of 

 12,554 feet, and may be considered as the main point of outflow. From 

 there the trachyte has spread over Carboniferous sandstones to the west- 

 ward, over Middle Cretacious shales to the east. Instances of intrusion 

 were observed at several points. One was noticed in particular where 

 the trachyte appeared as directly interstratified with red Carboniferous 

 sandstones.^ Near the top the intrusive sheets are wedge-shaped, but 

 lower down they closely resemble interstrata. I am inclined to the 

 opinion that we there have one of those instances that shows only a 

 small portion of the volcanic rock on the surface, while the main mass 

 remains buried. Considerable disturbances from the normal position 

 have been produced by this series of intrusions, although no striking 

 forms have resulted therefrom. So far as seen, there was but little dis- 

 turbance of the Cretaceous shales, owing, perhaps, to the fact that they 

 are a short distance removed from the centre of ejection. Much de- 

 nudation and erosion has taken place at that locality, and at some 

 points has brought to light portions of formerly hidden volcanics. 



At Station 36 the rock is very characteristic. A grayish-green micro- 

 crystalline paste contains innumerable crystals of opaque, white oligo- 

 clase, giving the trachyte the appearance of a very uniform pudding- 

 stone. Small acicular crystals of green and black hornblende are dis- 

 persed throughout the paste. A short distance from the station the 

 rock represents a type that is frequently found in connection with porphy- 

 ritic trachyte. An almost white micro-crystalline, feldspathic paste 

 contains minute crystalsoftransparentoligoclase,andvery small crystals 

 of a brown mica. It is highly probable that, were we able to go to any 

 considerable depth, we would find the oligoclase to be transparent 

 throughout. The opacity, which it shows near the surface is most likely 

 due to a partial alteration into caolinite. Oligoclase readily changes 

 upon exposure, particularly in regions of varying temperature. 



* Rep. U. S. Geol. Surv., 1875, p. 244. t Rep. U. S. Geol. Surv., 1874, p. 207. 



t Rep. U. S. Geol. Surv., 1874, p. 219. 



