| OCR Text |
Show 308 referable to the Cretaceous. These benches occupy too considerable areas, and Are too uniform in their relations to one another ( although slightly undulating, as though deposited upon an uneven surface), to admit of their present position being referred to the subsidence or settling of portions of the same basaltic level. Yet they may have originated in one and the same outpouring of molten matter, which wag deposited upon or spread over the inequalities of the surface, preserving, as it were, the topographical features wrought at the close of the Tertiary, and immediately preceding the epoch during which the present contours were fashioned, and which latter involved alike the wasting of the basaltic overflows and the old terrace- levels in the Tertiary and Cretaceous, which were flooded by the igneous matter. Besides these less conspicuous deposits, there is the great basaltic bed capping the Chicorica Mesa, in the midst of which there occur what appear to be distinct volcanic cones and craters of later origin. The Tanaja is probably an isolated remnant of the same great overflow, which involved an immense extent of territory along the eastern flank of the Spanish range, the actual extent of which may be inferred from the wide distribution of the comparatively insignificant areas which still exist in the mesas of Rayado and Gouzalitas, between the Cimarron and Ocat£, and probably other similar outliers farther to the southward. As has been suggested by Dr. Hayden, the origin of these immense outpourings of igneous products is probably due to immense and innumerable fissures in the earth's crust, the direction of which seems to have been determined by forces acting nearly at right angles to the old axis of upheaval in the range to the west. Through these fissures the molten matter escaped, overflowing extensive areas of the Tertiary plateau and perhaps lower levels of the Cretaceous, as would appear from the evidence afforded by the several distinct benches seen to- day in which these basaltic deposits vary in relative level at least 1,500 feet, comparing the extremes, as represented by the lowest bench on the borders of the Capulin Vega and the great bed forming the summit of the Chicorica Mesa* a few miles to the northwest. This great outburst of basaltic- making materials probably antedates the formation of the volcanic cones, which latter, it would appear, more properly pertain to a later time, or during the subsidence of the active period of overflow, representing, as it were, the expiring throes of volcanic action, where the forces were concentrated, and only manifesting their existence in isolated centers of true volcanic crater- building phenomena. When we come to review the few facts elicited during a flying visit, they seem merely to approach the threshold of a field the investigation of which would clear away all uncertainty respecting the origin and relations existing between the various and diverse phenomena there manifested. |
