Rhyodacite And Basalt

What the map says

Rhyodacite in sills between Lyons and Boulder is light-gray, very fine grained felsite and felsite porphyry. Sills are as much as 46 m thick, but most are about half that. Radiometric age (Larson and Hoblitt, 1973, p. 3) of Flagstaff Mountain sill is 64.

1:100,000 sheet: Colton, Roger B., 1976, U.S. Geological Survey Geologic Investigations Series I-855-G (15 crags). A contact line at that scale is about 100 m wide on the ground.

Served by Macrostrat (CC BY 4.0). The wording is the map's own, quoted as a fact about the mapped unit, not about any one wall.