GENERAL PETROGRAPHY 445 



laccoliths studied and described by Whitman Cross^*^ from Colorado, 

 Utah, and Arizona present many similarities. The Cerrillos Hills of 

 New Mexico are practically the same rocks, as described by Douglas W. 

 Johnson.^^ The resemblance to the laccolith at San Jose, Tamaulipas, 

 Mexico,^- is also striking and the association of soda-rich rocks is worthy 

 of remark. 



Structural and stratigraphical Eelations 

 general review 



West Butte, on the map, figure 3, is shown as one specially large con- 

 nected area of igneous rock, marked W. 1 and W. 2, and two other prin- 

 cipal outlying laccolithic masses, W. 3 and W. 4. Unfortunately, in the 

 preparation of this and the other geological maps, no topographic maps 

 were available. The laccolithic mass, W. 1, is shown in profile as viewed 

 from the southeast, in plate ?, figure 1. It is the loftiest summit in 

 West Butte and exhibits a general outline almost theoretically perfect for 

 a laccolith. Its northwestern extension, W. 2, is much lower. The rocks 

 are very similar in both exposures, and it may be that the two summits 

 are merely eroded parts of one great laccolithic mass. The steep escarp- 

 ment which faces east, on the southeast corner, and which is illustrated 

 l^y figure 2 of plate 7, is probably due to sheeting and weathering along 

 a fault-line. Glacial plucking has doubtless helped to accentuate the 

 relief, an inference strengthened by the incipient cirque shown in figure 

 1 of plate 8. On the south and west the dips of the fringing sedimentary 

 strata are so low as to lead one to infer that the laccolith was tapering 

 rapidly to its thin edge. The summits W. 3 and W. 4 are most probably 

 small auxiliary laccoliths outlying from the main mass. They are sepa- 

 rated from the main mass and from each other by depressions in which 

 Colorado shales still survive. The bases of the exposed laccoliths are 

 believed to rest on the Colorado shales, and the igneous rock at its intru- 

 sion presumably spread out laterally in this thick shale formation. Tlie 

 stratigraphical relations are shown by the section on figure 2. 



Middle Butte is contrasted with both the others in having its main 



1" Whitman Cross : The lax;colithic mountain groups of Colorado, Utah, and Arizona. 

 U. S. Geol. Survey, 14th Ann. Rept., pt. 11, 1894, pp. 165-241. 



11 D. W. .Johnson : Geology of the Cerrillos Hills, New Mexico. School of Mines Quar- 

 terly, vol. 24, pp. 173-246, 303-350, 456-500, and vol. 25, 1893, pp. 69-98. The la.st part, 

 on the petrography, is of especial Interest in this connection. The present senior writer 

 spent a week in the field with Dr. Johnson In 1892 and vividly recalls the similarities. 



■■2 J. F. Kemp : The copper deposits at San Jost5, Tamaulipas, Mexico. Trans. Am. 

 Inst, of Mining Engineers, vol. 36, 1906, pp. 178-203. 



.T. V. Kemp and G. I, Finlay : Nepheline-syenite area of San .Tose, Tamaulipas. Bull. 

 Ceol. Soc. Am., vol. 14, 1904, p. 534. 



