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The aphanitic microcracks inside the columns are small joint planes between the two crystal grains.
The trachyte occurs in two varieties, porphyritic and aphanitic, the latter of which is dominant.
The matrix is composed of an aphanitic quartz-feldspar mosaic.
Within 50 m of the main body, some centimetre-scale beds appear to have melted to thin layers of brownish aphanitic material which locally forms a selvedge around blocks.
At Ain Khuff the Khuff formation is 562 feet thick, with alternating lithology from top to bottom being as follows: (1) a 92.5-foot thick aphanitic layer of calcarenitic limestone, commonly marly and fossilifer-ous; (2) a 233-foot thick aphanitic limestone; and (3) a 110.5-foot thick layer of dolomite and shale, with angular granitic sand and fine conglomerate at the bottom.
The locality consists of broken outcrop and rubble of weakly cleaved, massive, light yellow-grey felsic tuff, aphanitic to sand-size and aphyric, of the Lower Birch Island Member, in a partly slumped half-metre wall along a roadside ditch on the south (uphill) side of the road.
The remainder of the formation comprises intensely recrystallized, black aphanitic siliceous rocks that contain fa int pink to grey siliceous blotches (relict fragments) up to 5 cm in length, which are probably siliceous lapilli.
At Ain Khuff in Saudi Arabia the Khuff formation is 562 feet thick, with alternating lithology from top to bottom being as follows: (1) a 92.5-foot thick aphanitic layer of calcarenitic limestone, commonly marly and fossi-liferous; (2) a 233-foot thick aphanitic limestone; and (3) a 110.5-foot thick layer of dolomite and shale, with angular granitic sand and fine conglomerate at the bottom.
The subvolcanic rhyolite porphyry has a red, aphanitic matrix and typically contains small rounded quartz phenocrysts and feldspar laths.