felsite

Felsite, fel′sīt, n. a fine-grained, compact rock, a variety of quartz-porphyry—also Fel′stone.—adj.

Felspar in Colorado and felsite magnesian slate in Newfoundland carry gold.

Other hard rocks such as chert, quartzite, felsite, granite, sandstone and volcanic rocks very frequently are largely represented in gravels, while coal, limestone and shale are far less common.

But felsite or microfelsite is still the generally accepted designation for that very fine-grained, almost crypto-crystalline substance which forms the ground-mass of so many rhyolites, dacites and porphyries.

Some rocks are felsitic in parts but elsewhere glassy; and it is not always clear whether the felsite is an original substance or has arisen by the devitrification of primary glass.