308 D. AV. JOHNSON VOLCANIC NECKS OF MT. TAYLOK E.EGION 



seen in tlie walls of both the Mount Taylor and the Prieta mesas. The 

 amoimt of dip is generally so small, however, that in a limited exposure, 

 the beds appear horizontal. In speaking of the exposures about the vol- 

 canic buttes, the imperceptible northward dip Avill be ignored and the 

 beds regarded as horizontal, except in those cases where local tilting is 

 evident. 



On this surface of faint relief was built the massive cone of the Mount 

 Taylor volcano and the surrounding lava flows. For detailed descrip- 

 tions of tlie main volcano, the lava flows, and the minor cones rising 

 above the flows, the reader is referred to Button's essay. According to 

 Dutton, the lava flows are not derived from the main volcano, but fi'om 

 numerous vents scattered over the tableland. Eepeated outflows, from 

 tliese vents interlaced to build up a lava cap several hundred feet in 

 thickness and some luindreds of square miles in area. Cinder cones Avere 

 built on the lava cap in places, indicating occasional explosive phases at 

 some of the vents. 



Thus closed the jjcriod of vulcanism, and lliere followed a long era of 

 quiet, during which we have no evidence of any recurrence of lava out- 

 breaks. With the exception of recent l)asalt fields farther south and 

 west, wliolly distinct from and not to be confused with the lavas of the 

 Mount Taylor and Prieta mesas, there is no trace whatever, so far as the 

 region has been studied, of any recurrence of volcanic activity since the 

 period of eruption Avhicli formed the great lava cap of the region. Con- 

 cerning the age of the flows which make up this cap, Major DuKon 

 writes (page 177) : 



"They were Terli;ir.v: probably Middle Tertiiiry. . . . None of them can 

 be regarded as recent in any sense wh;itever. Nowhere on the surfiico of 

 Mount Tayloj- or of its surrounding lava sho(>t lias nuy fresh-looking rock been 

 found. The traces of time are visible everywhere." 



Since the close of llie volcanic period, tlic bisloi'v of llie regiou has 

 been one of erosion. The coimtry of low reliet and the lava ca|) "which 

 overspread part of it Avere subjected to renewed dissection. Dee]) \al- 

 leys were cut through the lava cap into the underlying softer rock, and 

 extensive sapping of these softer beds all around the edge of the lava 

 cap caused continual decrease in the size of tlie mesa. The deepening 

 and })roadening of the Puerco valleYji as isolated theeast^ 

 mesa forming the smaller remnant called Prieta mesa. Today we find 

 that the lava cap and over 1,000 feet of the underlying sediments have 

 been removed over extensive areas, so that we recognize in Mount Taylor 

 mesa and its isolated remnants a tableland of sandstone capped by lava, 



