Pogue, Jr. — Geology and Structure of Volcanic Rocks. 237 



number of gabbro dikes were insinuated into the region. 

 These represent either the outliers of an independent gabbro 

 batholith or the differentiated off-shoots from a large mag- 

 matic reservoir of more acid nature ; and are probably related 

 to great intrusive masses of granitic, dioritic, and other mass- 

 ive coarse-grained igneous rocks occurring a few miles to the 

 west. 



The coming to place of the igneous masses, which adjoin 

 and probably undermine the district, inaugurated a period of 

 activity of circulation ; during which the rocks were mineral- 

 ized, in part by valuable ores, and universally by pyrite and 

 pyrrhotite ; many formations enriched " en masse " by silica ; 

 and a large number of quartz veins left along lines of major 

 circulation. The amount of material introduced seems to pre- 

 clude any but a magmatic source. It is not known to what 

 proportional extent, if any, the materials were contributed by 

 the gabbro or by the large granitic masses. From the great 

 amount of silica introduced, and from the acid character of 

 the gangues, it is likely that the granite was the dominant 

 source. 



The first event of practically known age is the introduction 

 of diabase dikes in Triassic time. There is no evidence for 

 estimating the length of the interval between the period of ore 

 deposition and the coming to place of the diabase. 



And finally, the forces of weathering and erosion, although 

 operative since the region was first elevated by folding, have 

 been especially active from the introduction of the diabase to 

 the present. This period then is dominantly one of planation 

 and rock decay : to such an extent, indeed, that the region has 

 been reduced once to an approximate base-level, and although 

 rejuvenated by uplift, is again approaching that state. 



Summary of geologic history. 



Pre-Cambrian (?) 1. Building up of the volcano-sedimentary 

 series. Alternation of volcanic activity and periods of 

 quiescence. 



2. Consolidation of the series. 



3. Operation of a compressive force, throwing the whole 

 formation into folds, and inducing schistosity, jointing, and 

 probably faulting. 



Paleozoic (?) 4. Approach of a mass of igneous rock, announced 

 by the insinuation of gabbro dikes into the region. 



5. Passage of solutions, depositing iron ores and silica, and 

 forming quartz veins and mineralized zones. 



Triassic, 6. Introduction of diabase dikes. 



Post-Triassic, 7. Period of weathering and erosion. 



