. Bulletin of the Geological Society of America. Geology. CROSS-SECTION OF SQUARE BUTTE. 407 Its petrographic character is similar to the socialite-syenite described later on. DIAGRAMMATIC SECTION OF SQUARE BUTTE. The facts which have been detailed in the foregoing pages may now be briefly recapitulated and summarized in the diagrammatic section shown in figure 6. If we were to pass a vertical axis through the center of the above sec- tion and revolve it upon this axis the figure of revolution which would be generated would represent quite correctly the structure of Square butte and the dispos
. Bulletin of the Geological Society of America. Geology. CROSS-SECTION OF SQUARE BUTTE. 407 Its petrographic character is similar to the socialite-syenite described later on. DIAGRAMMATIC SECTION OF SQUARE BUTTE. The facts which have been detailed in the foregoing pages may now be briefly recapitulated and summarized in the diagrammatic section shown in figure 6. If we were to pass a vertical axis through the center of the above sec- tion and revolve it upon this axis the figure of revolution which would be generated would represent quite correctly the structure of Square butte and the disposition of its several parts. Observe also in this connection the map on page i>t VERTICAL = HORIZONTAL SCM E up- white band; i = tran- Figure 6.— Cross-section of Square Buttt. a = white syenite ; b = dark basic rock; c = dark hoodoos : d = restored laccolitic cover turned Cretaceous sandstones ; f= protruding sheet or edge of laccolite ; h sition zone from white to dark rock, actual and imagined. SUMMARY OF FIELD RESULTS. From the facts thus shown we believe that Square butte is a laccolite consisting of two kinds of rock,'an inner mass of an acid feldspathic variety surrounded by a zone of a basic augitic one. That it is not a case of one intrusion occurring on top of another is clearly shown by the facts already presented, and by the further ones that the relations of the light rock to the dark one are in nowise determined by the varying topography, as must have been the case were the black one a lower in- trusive sheet, and by the inclined circular plane of the transition zone, which has approximately the form of the surface of a truncated cone. Basic peripheral zones in connection with intruded masses of igneous rock, caused by the local concentration of dark colored ferro-magnesian minerals, are known and have been described by several authors, but, so far as we have been able to discover, no example has ever been seen or described before which illustrates them
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