Lazvs of New Mexico Mountain Ranges. — Merrick. 305 
encounter the surface of the strata. The edg^es erode rapidly 
and the surfaces slo\vl\-. especially when a bed of limestone is 
reached. In no very long time the original escarpment has 
been moved from its original site and one might be tempted to 
ignore the causal connection between the faults and the es- 
carpment. Such a case occurs to the writer above the Graphic 
mines where soft shaly limestone is left in an abrupt escarp- 
ment from the base of which eastward to the base of the back 
slope proper there is a long level interval. This is one of those 
very prominent structural features that would have seemed to 
Mr. Johnson to illustrate his point with "diagrammatic clear- 
ness." In this case, the fault is there and a profound one; 
there is, in fact, a reversed-dip area, i.e. a thin portion dipping 
east mstead of west. A careful examination would convince 
any geologist that the escarpment is due to the faulting. The 
fallacy would be in supposing that the amount of the projecting 
escarpment would be the measure of the drop of the fault. 
The result would be the same if the drop w^ere very small 
provided the opportunity of differential weathering were af- 
forded.* 
But, as stated above, the Magdalenas are not a good illus- 
tration of block mountains. Mr. Johnson's critic ascribes the 
origin of the faults to the "attempt of the brittle limestones 
forming the back slope to adjust itself to a complexly warped 
surface produced by the major faulting." The case is not by 
any means so simple as this might be understood to imply. The 
limestones, quartzytes and shales of the Carboniferous and 
the sandy layers of the red-beds have suffered from a variety 
of disturbances. It is not yet possible to say with certainty 
at what geological period the major system of faulting oc- 
curred. W'e do know, however, that the great andesyte uplift, 
Which attended the origin and subsequent history of the primary 
volcanoes of the range, profoundly affected the stratified series. 
This is attested by the fact that the brecciated andesitic lavas 
near the sedimentaries are filled with fragments of the sedi- 
ments and of the granites underlying them, and that great 
masses of the Permian and other sedimentaries are caught up 
in and comminuted by these andesitic flows. These phenom- 
• The writer can recall no prominent structural feature in the area coTered 
by stratified rocks in the Magdalenas which cannot be traced more or less 
directly to the faulting. 
