PUBLICATIONS. 853 
the forms of erosion assumed by the rocks are shown in numerous 
illustrations. 
The lower half of the rocky slopes of the mountain consists of | 
dark-colored rock eroded into towers and spires, which are strongly 
contrasted with the light-colored upper half of the mountain, where 
the rock is in large masses and cliffs. [he white rock is sodalite- 
syenite, already described by Lindgren, the petrographical characters 
of which are briefly reviewed. The dark rock is a new type of rock 
consisting of much augite and less orthoclase, besides olivine, biotite, 
albite, anorthoclase and accessory nephelite, sodalite and other min- 
erals. The chemical composition of the rock, and that of the pyroxene 
are given, and the mineralogical features of the rock are fully described. 
The name shonkinite is proposed for the new rock. ‘The two rocks of 
the laccolite form one mass, erupted at one time; the marked differ- 
ences between them being the result of differentiation subsequent to 
their intrusion within the sedimentary rocks. The process of differen- 
tiation is discussed at length, and the opinion is expressed that no one 
simple process will explain all cases, but that a variety of factors must 
be taken into account, any one or all of which may operate to produce 
a given phenomenon. Wo'lee I 
The Laccolitie Mountain Groups of Colorado, Utah and Arizona. By 
WHITMAN Cross. Fourteenth Annual Report of the Director 
U. S. Geological Survey, for 1892-3. Washington, 1895. 
84 pp., 10 Plates, 19 Figures. 
Having become familiar with numerous instances of laccolitic intru- 
sions in Colorado, and having noted how much doubt concerning their 
true nature existed in the minds of some foreign geologists, Mr. Cross 
has undertaken to present the facts already known of such bodies of 
igneous rocks, so far as concerns their occurrence in regions explored 
by himself and in the neighboring regions of Utah and Arizona. And 
in so doing he has endeavored to establish more clearly the various 
phases of laccolitic intrusions within sedimentary strata, and to describe 
the petrographical character of the igneous rocks that constitute such 
intrusive bodies. The paper first reviews in considerable detail the 
facts established by Gilbert regarding the laccolites of the Henry 
Mountains, and the theory he advanced in explanation of them. It 
also reviews the characters of the rocks from a study of the specimens 
collected by Gilbert and originally described by Dutton. According 
