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quartz fragments from the Dagger Flat sandstone. The conglomerate also contains small angular pebbles of black chert, like that in the Maravillas itself, and blocks of fine-grained calcareous gray sandstone of an unknown horizon. Near Rock House Gap a boulder of the calcareous sandstone 5 feet long is embedded in the conglomerate (pl.5, B). At both localities the basal conglomerate contains abundant abraded corals and brachiopods. At Monument Spring fine conglomerates in the upper part of the lower member are interbedded with lenses and layers of black chert a few inches thick, which are clearly not transported and are probably not due to replacement of some previous rock (fig. 13, A). It is likely that they are chert deposits of primary origin.

The limestone layers above the conglomerates at Rock House Gap are very fossiliferous (pl. 5, A). The dense platy limestones contain orthoid brachiopods and graptolites, and the more granular layers, which form lenses, contain orthoid brachiopods and such trilobites as Cryptolithus. The shale zone near Monument Spring is an ashen-gray soft shale on which the dark-colored imprints of graptolites stand out with sharp contrast. A few feet from the top of the Maravillas at Monument Spring, in hard black shale partings between nodular cherts, a large graptolite fauna was collected.

-In the Dagger Flat anticlinorium the conglomerate beds of the Maravillas chert are fewer and thinner, and the differentiation between the lower calcareous and the upper cherty parts of the formation is not sharply marked. Near the middle of the formation along the southeast flank of the anticlinorium there are reef-like aggregates of bryozoans, some of which form massive beds with a maximum thickness of 10 feet. These are absent in the northwest flank of the anticlinorium and farther to the northwest. There are also some massive limestones with very irregular bedding, evidently having a channel structure. They contain a few comminuted fossils. The following section (sec. 13, pl. 2) of the Maravillas chert is characteristic of the southeastern exposures of the formation:

-In the southeasternmost exposures of the formation chert greatly predominates over limestone. In the Santiago Mountains, 8 miles south of Maravillas Gap (Santiago Peak quadrangle), it is nearly all black chert with a few thin lenticular limestone beds. On Rough Creek, in the Dove Mountain quadrangle, it consists of black chert with bands of gray chert in 3-inch to 1-foot beds and with two or three layers of black limestone. In this region there is a few feet of dark-green shale between the uppermost cherts and the base of the Caballos.

Thin sections of the cherts from the Maravillas formation show them to consist of a fine-textured mass of