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Show 285 regular than is usually observed, and near the middle occurs a sort of miniature crater, or " blow- out", as these phenomena are familiarly termed here. The annexed diagram indicates the relation of the crater* vent ( a) to the main dike. ( Plate 43, Fig. 1.) Here the metamorphism is also very apparent, the shales being changed to slate a distance of twelve to twenty inches either side of the dike. Following down the arroyo a short distance southward, the low blufffc immediately bordering the north bank of the Cimarron are reached, and which are here capped by a bed of basalt two to four feet thick. This bed seems to conform to the plants of stratification of the Cretaceous shales, which in places overlie the intruded igneous deposit, and which are changed for a few inches in depth above and below. In the main, this bed is quite regular, extending above half a mile in the face of the bluff, at the upper end of which it has been wrought for building- material. But at one point itseemsto be connected by a sortof " blow- out" with another and higher bed, which latter occupies a position fifteen to twenty feet higher, ontcropping at the surface over a considerable extent, but overlaid by seventy- five feetor more of shales in the mesa at the lower extremity of the bluffs. The igneous matter of these basaltic beds appears to have followed the planes of stratification in the Cretaceous shales, so that their outcrop in the face of the bluff has all the appearances of a sedimentary deposit; and, but for the nature of its composition and the prevalence of minute crystals of quartz, and perhaps one or two zeolites, together with the singular concentration or " blow- out" of molten matter by which some sort of connection is established between the two masses, their true character might be entirely overlooked in a cursory examination. Whether the horizontal fissures have any connection with the beforeraentioned vertical dike could not be ascertained ; but that they owe their origin to some such relationship hardly admits Of a doubt. THE TERTIARY PLATEAU. Occupying the angle between the main range on the west and the Baton Hills to the north, comprising an area of about six hundred square miles within the borders of New Mexico, occurs the great Lignitic formation of the Tertiary. This formation, in consequence of its vast mineral wealth, consisting of deposits of coal and iron, has received perhaps even more critical examination than has been bestowed upon the older sedimentary formations with which it is associated throughout the region south of the Colorado divide; and the little we may add from the present examinations is rather designed to show its extent in this particular region than further details relating to its stratigraphy. The eastern border- limits of the formation appear in the line of lofty bluffs which flank the western margin of the basin of the Canadian, extending from the Baton Pass in a southwesterly direction above forty miles to the Cimarroncito; thence stretching northwesterly some ten or fifteen miles to Ute Valley. To the south of the Cimarron Biver, the formation occupies a narrow belt, where the strata show apparent signs of having participated, at least to a slight degree, in the uplift parallel to the Urac ridge, a spur extending eastward from the Black Mountain group and terminating in the plain between the Cimarroncito and Urac Creeks, forming a lofty granitic barrier 2,000 to 2,500 feet above the plain, and which defines the southern limits of the Baton coal- field. The rounded condition of the quartz particles in the granitic mass of |
