Art. IV.] Herrick, Geology of New Mexico. 83 
a fracture zone in which lie the chief centres of overflow. Else- 
where the greater igneous activity is on the eastern side though 
just what the determining circumstances were we are as yet un- 
able to say. It is a well-known aphorism that the crests of 
greatest elevation, by reason of greater fracture and superior 
accessibility, become sooner or later the depressions and, ceteris 
paribuSy the courses of the streams. Our area is no exception 
and the moutain ranges of the west bank of the river are con- 
sidered to be secondary in the sense that they are, in part, the 
remnants left by erosion from the original uplift. To the preser- 
vation of these ridges the frequent and often extensive outpour 
of eruptive rocks has greatly contributed. At first it might 
seem that the vent for such overflows should have developed in 
the axis of the fold but the analogies as well as a variety of 
theoretical considerations reinforce the other view and sustain 
the evidence from the field. Thus the shear strains and the 
thickening and heat increment would be greatest near the sides 
at the base and the isogeotherms would accordingly be elevated 
at this place. ^ There is much reason to suppose that the ma- 
terials of the eruptives are essentially of local origin, in fact 
that they are derived from the fusion of materials lying at no 
great depth beneath the site in which they now occur. The 
agencies of metamorphism being then greatest at the sides near 
the base and the techtonic strains there having a maximum it 
follows that the lower portion of the strata are there fused, the 
support for the rest is removed as a result, there is a faulting 
of the whole, causing the tilting of the series toward the axis 
of weakening on the “down” (here western) side and an out- 
pour of fused material at the same time. If the thrust and out- 
flow were violent there would be a second fault of the same sort 
to the west. This has apparently been the history of our re- 
gion. The limestone occupying the upper part of the series is 
relatively infusible while the subjacent acid rocks consisting of 
sandstone and conglomerate was readily altered— in some cases 
1 Hot springs and secular vibratious §till attest the elevation of the isogeo" 
thevms in this belt, 
