400 WEED AND PIRSSON — HIGH WOOD MOUNTAINS OF MONTANA. 



sections, show them to be of novel types and of great interest from a 

 petrologic standpoint. This is so especially true in the case of Square 

 butte that we have already made a detailed study of the facts presented 

 in this particular area and the results form part second of this paper. 



Part II. — Square Butte and its Remarkable differentiation Zone. 



INTRODUCTORY. 



The relation which Square butte bears to the Highwood range has 

 been briefly mentioned in part I of this paper, and it is clearly shown 

 on the map on page 393. The range consists of a number of closely clus- 

 tered peaks with two very conspicuous detached elevations, the larger 

 and more easterly of which is Square butte. Although it is genetically 

 and geologically part of the Highwoods, it is geographically a distinct 

 and separate elevated mass. It is the most prominent landmark to be 

 seen from the level lands which stretch far to the north and east, and its 

 dark base and white crown make it conspicuous at a distance of 15 or 

 20 miles. The name it bears is of course derived from its flat top, which 

 distinguishes it so sharply from the pointed peaks of the Highwoods. 



Square butte was visited by the authors during the past summer, while 

 studying the geology of the Highwood range, and found to possess cer- 

 tain peculiarities of structure and origin which render it, from the stand- 

 point of broad petrologic geology, one of the most interesting occurrences 

 of igneous rock which has ever been described. 



HISTORICAL. 



The only published account of the butte is in a paper by Mr W. Lind- 

 gren and the late Dr W. H. Melville.* In this paper, the butte is de- 

 scribed as being composed of a light gray eruptive rock, having a distinct 

 lamination, with several horizontal sheets of a dark volcanic rock intruded 

 in the sediments around its base. Specimens of the light colored rock 

 forming the upper portion which had been collected by Dr C. A. White 

 were subjected to a very complete petrographic and chemical study, from 

 which it was shown to be a peculiar member of the nephelite-syenite 

 family of igneous rocks, consisting chiefly of a black, soda-rich horn- 

 blende (near barkevikite in composition), alkali feldspars and sodalite ; 

 in short, a sodalite-syenite. No other reference to the butte has been 

 found by us in the literature. 



GENERAL DESCRIPTION OF SQUARE BUTTE. 



Square butte is a rudely circular mass resting on a nearly level table- 

 land, which has an elevation of about 4,000 feet above the sea. This 

 platform consists of nearly horizontal sandstones and shales belonging 



*Am. Jour. Sei., vol. xlv, 1893, p. 286. 



