133 Microscopic Petrography – The Younger Dacite Flows

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography

 

The Younger Dacite Flows

The lavas discussed under this caption are those erupted as domes and flows from the Northern Arc of Vents, namely, those of Llao Rock, Grouse Hill, Cleetwood Cove, and Redcloud Cliff. With these are included the lavas of Cloudcap and Scott Bluffs.

Among the features that serve to distinguish the dacites of the Northern Arc of Vents from the older southern dacites, the chief is the fact that the glass of the younger lavas is rarely devitrified. A second distinction is the much greater development, especially at the tops and bottoms of the flows, of black obsidian. Thirdly, pale-gray pumiceous types of lava are more plentiful among the older dacites. Fourthly, the dacites of the Northern Arc are in general much poorer in tridymite. And finally, though they are never rich in hornblende, few are entirely without this mineral. Many of the lavas of Cloudcap and Scott Bluffs, on the other hand, closely resemble the southern dacites, and some are hardly to be distinguished from acid andesites.

 

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