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scarcely two dikes having precisely the same mineral constituents and 

 aggregation, though all agreeing in their more or less somber hues. 

 Practically vertical, in several instances a slight northerly iuclinatiou 

 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 mineralogical combina- 

 tions 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 

 PoQil, a small dike may be traced up through the Cretaceous shales into 

 the overlying Tertiary sandstone, where it is lost to view in the debris 

 accumulated upon the slope. Mr. Morley has observed in the caiion of 

 the Canadian, in the vicinity of the Eaton divide, a similar dike, con- 

 nected 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 metamorphism 

 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 metamorphic 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 con- 

 verted 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 organ- 

 isms 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. Arms'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 being attenuated, the interven- 

 ing 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 meta- 

 morphic action extending three to five inches either side. The shales 

 are charged with a species of Inoceramus an(J 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 aloug the north side of the stream, and 

 which is here interrupted by a shallow arroyo, a narrow dike, averag- 

 ing perhaps two feet in thickness, may be traced upward of three hun- 

 dred yards in a nearly east-west direction. Its course is less direct or 



