DAKOTA FOKMATION. 63 



of hard, white, slaty shales, 10 to 30 feel thick, transitional to the Benton; 

 and a fossil flora is Found throughout. 



sandstones. — These a re t 'i n ii ) i< isei 1 chiefly tit quartz, hut contain a trace 

 of mica and a small amount of iron which stains their normal white a 

 brown, yellow, or red. Bitumens are locally present, which also imparl 

 a brown color- Cross-bedding and ripple-marks are common features. 

 The sandstone is harder and more compact than any other Mesozoic or 

 Tertiary rock of the field, and now and then verges upon quartzite, espe- 

 cially in the upper bench. It can hardly have been submitted to greater 

 pressure or heat than the more loosely agglomerated Triassic sandstones, 



and its hardness may therefore be attributed to a i -e than usually large 



amount of silica in solution in the sea waters in which it was laid down. 

 The conglomerate at the base of the formation is especially compact 

 through silicification, fracturing across pebbles and matrix alike. A pecul- 

 iar feature of certain layers of the sandstones is the agglomeration of the 

 grains into forms which resemble short pieces of spaghetti from a half inch 

 to 2 inches in length, with rounded ends, and woven in and out in the most 

 irregular manner. To what these forms are due it is impossible to say, but 

 they suggest the casts of worm burrows. 



conglomerates. — The conglomerate at the base of the Dakota is char- 

 acteristic. It varies from a thin, almost imperceptible layer to one 30 feet 

 in thickness, and is composed of well-rounded, smooth, in some cases 

 almost glazed, pebbles from the size of a pea to a diameter of 1 inch. 

 The pebbles are derived from most of the older formations down to the 

 Archean, including some of which no trace has yet been discovered in this 

 field. Among the latter are a few of Silurian age, identified by contained 

 fossils, and others carrying small corals of undetermined affinities; in others 

 silicification is so advanced that their original composition is too much 

 obscured to permit determination of their geological source. The pebbles 

 comprise abundant limestones (some closely resembling those of the Trias), 

 quartzites, clays, flints, jaspers, and rocks of granitic composition, together 

 with the separate mineral constituents of the last. 



Besides the basal conglomerate, a thin sheet containing very minute 

 pebbles is sometimes found at the base of the upper bench of sandstone, 

 and a third near the summit of the formation. 



