250 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



II.— ANARHACTIC. 



Two regions, mainly, may be distinguished where anarhactic erup- 

 tions of basalt occur. Of these the first is in Northern Colorado, north 

 of White River. Numerous dikes of basalt there traverse Cretaceous 

 and Tertiary strata. They frequently extend in straight lines for a 

 number of miles. Analogous to the occurrences of porphyritie tra- 

 chyte, the formerly inclosed basaltic rock has been exposed by the abra- 

 sion of the surrounding sedimentary beds. Vertical black walls now 

 stand out prominently, imparting to the landscape a unique character 

 that has found its appreciation in some of the local names given. Es- 

 sentially these dikes are parallel among themselves, often separated by 

 considerable intervening spaces. 



On the northern tributaries of thy San Juan River a large number of 

 basaltic dikes occur in the Tertiary beds of that region. Along the San 

 Juan itself sucih dikes may be found. Frequently they extend for many 

 miles, exhibiting the same characteristics above given. 



A marked feature in connection of these dikes is the fact that no dis- 

 turbance of the strata seems to have accompanied the ejection of the vol- 

 canic material. No distortion or extensive displacement of the beds 

 will be noticed. Evidently the fissures were formed by the action of 

 some force that confined itself to this manifestation. Through the open- 

 ings produced the lava protruded, without causing any appreciable fur- 

 ther disturbances. Rising into the fissures, and sometimes overflowing, 

 the lava obeyed the laws governing its eruption without any accompa- 

 nying demonstration of additional force. 



This characteristic brings the anarhactic into close relation with the 

 massive eruptions. The two stand as the representatives of definite 

 types, while the ejections of porphyritie trachyte were preceded and 

 accompanied by more violent demonstrations. It is true that a certain 

 amount of disturbance must have been coincident with the opening of 

 the fissures ; but it is equally true that, whatever the acting force was, 

 it did not exert any considerable iufluence upon the rocks adjoining. It 

 is evident that this feature is one of great importance in discussing the 

 origin of eruptives and their mode of rjectiou. It is necessary, there- 

 fore, to note the conditions pertaining to the surroundings of erupted 

 masses, as well as those relating directly to the latter. 



III. — ISOLATED. 



A number of isolated basaltic eruptions occur in Colorado. Promi- 

 nent among them is that of Golden City.* Table Mountains there are 

 composed of lignitic beds and covered by basalt. Mr. Marvine says 

 (loc. cit.) with regard thereto: "The source of this lava is from beneath 

 North Table Mountain, on the summit of which, and near the northwest 

 corner^ the remnants of a group of small volcanic cones may still be 

 seen ; weather-beaten and nearly worn away, they still suffice to show 

 from whence the lava came." This explains, in a few words, both the 

 source of the basalt and the character of such eruption. Not far from 

 Golden, at Valinont, a heavy dike of the same material may be oserved. 

 Inasmuch as we may safely regard isolated eruption as the results of 

 local dikes that have overflowed, that of Valmont deserves mention 

 here. 



*Kep. U. S. Geol, Sarv., reprint, 18G7 to 1869, p. 133, and Rep. 1873, p. 129. 



