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 bluifs 

 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 planes of stratification of the Cretaceous shales, 

 which in places overlie th-e intruded igneous deposit, and which are 

 changed for afewinches.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 it seems to be connected by a sort of " blow-out" with another and 

 higher bed, which latter occupies a position fifteen to twenty feet higher, 

 outcropping 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 sedi- 

 mentary 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 mat- 

 ter 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 beforementioned vertical dike could not be ascertained ; but 

 that they owe their origin to some such relationship hardly admits of a 

 doubt. 



THE tehtiary plateau. 



Occupyiug the angle between the main range on the west and the 

 Eaton 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 i:>articular 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 i3asin of the Canadian, 

 extending from the Raton 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 Eiver, 

 . the formation occui)ies 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 Eaton 

 coal-field. 



The rounded condition of the quartz particles in the granitic mass of 



