Cumberland Bay Series: Upper Division.—This consists largely of hard, coarse, compacted crystal tuffs, dark-grey or black in hand specimens, and resembling coarse grey wackes. In thin section they contain fragments of trachytic lavas, and detached crystals of orthoclase and oligoclase in a paste of minutely crystalline material mixed with much indeterminate matter. Iron-ores and ferromagnesian minerals are rare. In some of the rocks andesitic and latitic fragments are dominant, and occasional fragments of yellowish glass enclosing euhedral orthoclase are seen. Many of the rocks have suffered some degree of scapolitization. The tuffs pass into siliceous grey wackes still with some recognizable volcanic material, and are interbedded with fine banded mudstones which appear to represent volcanic muds. Radiolaria have been recognized in these rocks as well as in the tuffs. The mudstones occasionally become somewhat laminated and pass into black shales. An imperfect cleavage traverses many of these rocks at a high angle to the bedding. These tuffs doubtless represent the interbedded ‘diabas - schalstein’ of Thürach.