AveusT 9, 1918] 
A number of facts militates strongly against 
the Henry Mountains explanation of locco- 
lithic protuberance. Three basic premises ap- 
pear wholly untenable. Most vitiating is the 
seeming incompetency of simple hydrostatic 
pressure to produce the desired results. In- 
adequacy of relative lithologic density is now 
commonly conceded. There also appears to be 
a radical disparity between the physical con- 
ditions accompanying the formation of lacco- 
liths and their once supposed nearest kin the 
sills. : 
On the other hand the recent unearthing of 
the infrabasal make-up of certain laccoliths 
clearly points to a fundamental dependence of 
this class of mountains upon prior geologic 
structure. The shape of laccolithic masses is 
found to be cuneiform instead of lenticular; 
and thus at once does away with the blister 
idea. Quite essential appears to be the pres- 
ence of crustal lines of weakness. The mag- 
matic swelling or localization of laccoliths is 
discovered to be a direct function of oro- 
_ graphic potentialities. 
In seeking an immediate cause for his lac- 
colithie intrusion Professor Gilbert did not 
lose sight of certain mechanical shortcomings 
of his explanation. These he sought to over- 
come by appealing to certain associated fac- 
tors, which, however, later, Doctor Cross 
showed to be both unnecessary and not demon- 
strated as such. Professor J. D. Dana got 
over the difficulties by brushing aside all con- 
siderations except simple hydrostatic pressure 
and with this feature alone regarded the 
Gilbertian hypothesis complete. This is doubt- 
less one of the main reasons why from a 
mechanical angle leading European geologists 
have so persistently challenged the American 
view of laccolithic intrusion. At the same 
time Old World writers on the theme offer no 
alternative theory to take the place of the one 
which they seek to discredit. Through the 
results of close inspection of certain laccoliths 
of northern New Mexico the chief objections 
which were raised against the Gilbert view 
seem to be fully met. Controlling tectonic 
factors which all describers of laccoliths have 
SCIENCE 
139 
missed thus appear to supply the long sought 
desiderata. 
As a primary consideration in order that a 
laccolith be produced rather than any other 
form of volcanic manifestation it appears that 
the intrusive mass shall have a particular tec- 
tonie setting. Profound faulting is one of 
these prime factors. Another is orographic 
flexing by which the rigidity of certain arch- 
ing strata largely maintains the load of super- 
incumbent materials. Probably the high vis- 
cosity of acidic magmas has an important 
but as yet uncalculated influence on events. 
The remarkable infrabasal structure which the 
New Mexico laccoliths reveal carries the in- 
quiry a step more remote and explains the 
deep-seated cause of the major faulting, 
whereby an orographic prism is sustained by a 
sharp Pre-Cambrian arch, the rigidity of 
which is not even yet lost although the adjoin- 
ing blocks on either side are allowed to slide 
down, as it were, the steep sides of the old 
flexure. 
Now at the southern terminus of the Rocky 
Mountain Cordillera, in northern New Mex- 
ico, there is a succession of open flexures, the 
amplitude of which grows less as they recede 
from the main axis. It is where these folds 
eross great fault lines that laccoliths form. 
Thus through direct mathematical analysis of 
the tectonic problems presented and in the 
satisfaction of the most urgent tectonic de- 
mands an adequate raison d’étre for laccolithic 
genesis and location seems to be offered. 
CuarLes Keyes 
SOIL REACTION AND THE PRESENCE OF 
AZOTOBACTER 
Durine the summer of 1917 the writer con- 
ducted a preliminary survey of local soils to 
ascertain the relative nitrogen-fixing ability 
and prevalence of Azotobacter. Ninety soils 
were collected within two miles of the lab- 
oratory. The samples were taken from as 
widely varying soil conditions as could be 
located including the following: cultivated, 
permanent alfalfa, bluegrass sod, native pas- 
ture, barren hilltops, river bottom, sand bar, 
roadside and forest. 
