Abstract

In an area on Gardiner River, a flow of rhyolite followed a small valley previously eroded in a basaltic surface. Remarkable contact effects were produced.

In many places the rhyolite penetrated deeply into the basalt as complex networks of veins and dike-like bodies, which ramify through the basalt in intricate patterns. Corrosive action of the rhyolite upon the basaltic surface carried away material bodily and effected changes in the original channel. The composition of the basalt near contacts was greatly modified. Basaltic constituents were removed and rhyolitic constituents were substituted in such a manner that the compositions of altered rocks lie on straight lines between basalt and rhyolite.

The facts of occurrence are clear. In attempting to explain them it has seemed necessary to attribute properties to the rhyolitic magma somewhat at variance with those usually postulated.

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