IGNEOUS FORMATIONS. 297 



QUARTZ- PORPHYRY. 



Some very limited dike occurrences of a quartz-porphyry in the 

 vicinity 61" Boulder have been described by Prof. C. S. Palmer. 1 They 

 were no1 observed in the field work for this report, ;in<l are too small to be 

 represented upon the map. 



SECTION II. PETBOGBAPHICAIi DESCRIPTION. 



INTRODUCTORY REMARKS. 



It is tin- intention to presenl in the following sections petrographical 

 descriptions of the more important and interesting rocks of the district 

 The more common sedimentary rocks and the Archean schists and gneisses 

 arc omitted from consideration, the former because their character has been 

 sufficiently brougb.1 ou1 in the discussions of formations, the latter because 

 it was not within the scope of this report to study the mountain area, and 

 the types of these rucks thai have been observed are so simple and common 

 that there is no necessity for detailed description. 



The strata of the Denver formation are of peculiar interest both 

 geologically and petrographicallv. The recognition of this important 

 formation was originally due to its lithological character, and the materials 

 of its sediments afford the only known evidence of a period of remarkable 

 volcanic eruptions from a center not far distant. These rocks are therefore 

 described in detail sufficient to demonstrate their peculiar character. The 

 massive eruptive rocks naturally require description to bring out their 

 characteristics. 



BASALT. 



The various masses of basall occurring in the Denver Basin represent 

 the structural modifications which have resulted from the differences in the 

 conditions attending the consolidation of a certain magma 



The large dike of doleritic basall at Valmont represents the magma 

 cooling in the eruptive channel, probably 2,000 feet or more from the 

 surface of the time of eruption. The Ralston and adjacent 'likes and 

 masses must have cooled nnder approximately the same circumstances as 

 the Valmont rock. In the capping sheets of Table .Mountain is seen the 



The quartz-porphyry of Flagstaff Hill. Boulder, I olo. : l'roc Colo. Sci. Soc, Vol. III. p. 351, 1891. 



