236 jPogue, Jr. — Geology and Structure of Volcanic Rocks. 



to great thickness deposits of tuffs and breccias ; and periods of 

 comparative quiescence, accompanied by some weathering and 

 erosion and the deposition of the slates. Between successive 

 outbreaks, the magma probably underwent a certain degree of 

 differentiation ; so as to give rise to acid rocks at one time, 

 and comparatively basic rocks at others.- It seems evident 

 that there were frequent swings between two not very diverse 

 extremes, and that at no time did the product dejDart far from 

 the average type — a rather acid rock high in soda. Perhaps 

 each important outbreak poured forth rhyolitic, intermediate, 

 and andesitic materials. 



It would be impossible to picture the details of this volcanic 

 activity. It is suggested, however, that the outbreaks were 

 largely eruptions along fissures, breaking up through the series 

 of already formed horizontal rocks at frequent points in the 

 entire volcanic region. 



All of the slate and much of the fine tuff give evidence in 

 bedding planes of deposition by water. The coarse tuffs and 

 breccias may be air-laid or water-laid, or both. The flows 

 may have taken place upon the surface of the land or under 

 water. Possibly the entire series represents an off-shore 

 deposit, with submarine volcanic activity alone or accompanied 

 by outbreaks upon the shore. Or the region may represent a 

 river flood-plain or delta deposit. 



It is believed from chemical evidence that the slate material 

 was transported from no great distance. Hence a probable 

 view is to consider the volcano-sedimentary series a basin 

 deposit, the material for which was derived from beneath an 

 area of limited extent, and the thickness of which was limited 

 only by the depth of the magma reservoir and the amount of 

 material extruded. Thus by isostatic sinking of the crust 

 block capped by a layer of sedimentary and volcanic rocks, as 

 more material was forced up through it and deposited upon 

 its top, a series of great thickness could have been formed, 

 without drawing materially upon the surrounding country for 

 sediments, and conversely without bestowing evidences of its 

 nature upon regions not within its own confines. 



However laid down, the tuffaceous and sedimentary rocks 

 must have undergone cementation or consolidation before they 

 were capable of being thrown into folds. This process doubt- 

 less accompanied the formation of the deposits. 



No evidence is afforded for estimating the length of time 

 which intervened between the formation of the rock series and 

 its folding. It may be that the compression put an end to the 

 constructive epoch ; or it may equally be that this force was 

 long deferred. 



Following the folding after an unknown interval, a great 



