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Show 284 scarcely two dikes having precisely the same miueral constituents and aggregation, though all agreeing in their more or less somber hues. Practically vertical, in several iustauces a slight northerly inclination from the perpendicular has been noted, while an opposite pitch has not been observed, although such may occur. These miniature dike exhibitions offer a most interesting field for investigation, both on account of the variety in miueralogical combinations as also the phenomena connected with their occurrence. Although more apparent in the plains where the soft shales have weathered away, leaving the more rugged basalt standing up in long ridges above the surface, strongly contrasting with the prevalent green and drab herbage tints, they are probably equally well represented in the Tertiary plateau. Indeed, in the great slide in the point of bluffs between the Cimarron and Ponil, a small dike may be traced up through the Cretaceous shales into the overlying Tertiary sandstone, where it is lost to view in the dibm accumulated upon the slope. Mr. Morley has observed in the caiion of the Canadian, in the vicinity of the Baton divide, a similar dike, connected with which he reports most interesting phenomena. The dike is thrust up through the Tertiary strata, and at the locality referred to, it is exposed at a point where it crosses a thin bed of lignite, which it has converted into graphite. The extent of this intense metamorphiBm was not ascertained; but it is probably limited to a few inches either side of the protruded igneous matter, unless the dike be a large one, in which case its metamorpbic action would be proportionately extended laterally. Wherever these protrusions of igneous matter are favorably observed, they are seen to have exerted to a greater or less degree meta-morphic action in changing the immediately contiguous strata through which they pass. Thus, in the Cretaceous deposits, the shales are converted into tough slate, the effects of the metamorphism gradually diminishing the farther removed from the originating* cause. When the dike crosses a calcareous deposit, the intense heat has had the effect, to a limited extent, of partial calcination, by which whatever organisms it may have contained are obliterated almost past recognition, while the mass of the rock is more or less hardened. An instance where the lateral extent of metamorphic action may be readily examined occurs in the foot of the little mesa near Mr. Arras's, on the west side of the Canadian. At this locality a couple small dikes protrude above the surface, forming miniature parallel ridges about eighteen yards apart, with a strike nearly east and west. They vary somewhat in width, averaging about fifteen inches, and nearly vertical. At frequent intervals, they are interrupted by abrupt fractures, sometimes resulting in an oblique break; in other instances, the dike is completely dislocated, the overlapping extremities beipg attenuated, the intervening shale much broken and polished, with slickenside surfaces. The shales otherwise exhibit no indications of disturbance, the igneous mat-ter having been admitted through a simple fracture or parting of the strata. Immediately contiguous to the dike, the shales are much altered, becoming very hard, and showing distinct cleavage- structure, the metamorphic action extending three to five inches either side. The shales are charged with a species of Inoceramus and fish- scales, which occur equally abundant in the hard . metamorphosed portion. Perhaps three miles below Cimarron, in the interval between the high terrace- mesa which extends along the north side of the stream, and which is here interrupted by a shallow arroyo, a narrow dike, averaging perhaps two feet in thickness, may be traced upward of three hundred yards in a nearly east- west direction. Its course is less direct or |
