CROSS-SECTION OF SQUAHi: BUTTE. 407 



Its petrograpliic character is similar to tlie sodalite-syenite described 

 later on. 



DIAGRAMMATIC SECTIOX OF SQUARE BUTTE. 



The facts which have been detailed in the foregoing pages may now be 

 briefly recapitulated and summarized in tlie 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 (juite correctly the structure of Square butte 

 and the disposition of its several parts. Observe also in this connection 

 the map on page 402. 



_J VERTICAL « HORIZONTAL iCAL E 



Fi.;uuK G. — Cro.-ii-.stcliun of Square Buttt. 



a — white syenite ; b = dark \n\-^\c rock ; c = dark hoodoos ; d = restored lacoolitic cover ; c = up- 

 turnetl Cretftceous sandstones ; f— protruding sheet or edge of laccolite ; h = white band ; i = tran- 

 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 U)[) 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 with such striking completeness 

 of process and such perfection of erosi^'c dissection as Square butte. 



The significance of the Axcts and tlieir bearing on tlicoretic petrology 

 will \)G discussed in the latter [jortion of this jiaper. 



I'ETliOaRAI'IlY OF SQUARE BiTTE. 



Character islirs and Minerals nf the dark Rock. — Megascopic and Micro- 

 scopic. — Tlie dark rock, seen at a distance, appears of a grayish black or 



