LOCATION, EXTENT AND TOrOGIlAPlIY. 391 



by liindgren"^ give the results of very careful and detailed petrograpbic 

 studies of several rock specimens from this field. Tbese studies sbow 

 tbat from a jietrologic standpoint tbe rocks of tbis district are of great 

 and unusual interest. 



GEOGRAPHY ASD TOPOGRArHIC CHARACTERISTICS. 



The Iligbwood mountains form a distinct unit in tbe geography of 

 Montana. Tbe front of tbe Rocky mountains, formed of tbe Little I^elt 

 range, lie to tbe south of tbe group, with a l)road, flat and open valley 

 between, while tbe Judith and Big Snowy ranges fill the horizon to tbe 

 northeast. Standing alone and rising abruptly from tbe flat Cretaceous 

 plains, the mountains possess an imposing appearance bardl}^ warranted 

 by their height. The 800 square miles of highland forming tbe group is 

 some 25 miles long from east to west, witli a breadth, or north-and-soutb 

 direction, of from 15 to 10 miles. The loftiest jieaks, Higliwood and Ar- 

 row, reach a height of 7,GO0 feet above tide, or 4,000 feet above the })lains 

 which swee}) away in monotonous continuity from the foot-slopes. 



From most points of view the mountains are seen to consist of a gently 

 contoured, rounded group of comparatively low peaks forming the north- 

 ern half of tbe group, rising gradually and culminating in tbe higher 

 rugged crests which constitute the southern i)art of the cluster. A strik- 

 ing feature is a deep cut, drained l)y Arrow and Highwood creeks, which 

 separates the mountains into east and west portions and forms a pass 

 known as Highwood gap. While the main mass consists of an irregular 

 collection of peaks connected by high ridges which make a topographic 

 whole, there are also two buttes l^'ing east of the mountains which are a 

 part of the grou):). Tbe easternmost is a flat-topped mountain mass, rising 

 to an elevation of nearly 6,000 feet and forming the most salient feature 

 of tbe landscape wherever visil)le. This is Square Initte, an eminence as 

 important geologically as it is prominent topographically. Between this 

 and the main ridges of the mountains rises the dark pillared form of 

 Palisade butte, the other of the two. 



Surrounding tbe range on every side is the open i)lains country, cut 

 into picturescpie badlands along the Arrow river, but elsewhere preserv- 

 ing tbat j)ersistent horizontaHty so characteristic of the CJreat i)lains of 

 the northwest. Rising aljruptly from tbis level area, the mountains act 

 as local condensers, so tbat the slopes are well watered ; tliunderstorms 

 are common during tbe summer, and tlie higher i)eaks generally cloud- 

 enveloped for a large partof tlui year. Although notof suflicient height 

 to maintiiin j)ermanent snow l>;inks, th(! mountains give I'ise to several 

 fine streams, which head in springs far up tbe narrow V-yh^i-pcd trenches 



•"Eruptive Rocks from Montanii." Proc. Culiforriia A<-a<l. S<i., Her. •_', vol. iii, 1890, p. 39; niso, 

 Am. Jour. Scl., toI. 45, 189:J, p. 2SC. 



