3810 Weed and Pirsson—Igneous Rocks of Montana. 
piles that obscure all exposures. Dikes are occasionally found 
in the foot-hills or surrounding plains country, and appear to 
have a direction radiant from the higher peaks. 
The East Butte, whose highest peak reaches an elevation of 
6,200 feet above the sea, consists of four main points arranged 
in an irregular square. The conical northwestern peak is the 
most prominent; it is connected by a high ridge with the 
round-topped southwestern peak. The sedimentary rocks con- 
sist of yellowish and gray sandstones and black shales of Colo- 
rado Cretaceous, or Belly River (Montana?) age, the latter 
rocks being carved along the valleys into castellated shapes 
whose fluted surfaces, baleony ledges and intricate fret work, 
form most picturesque and attractive forms. These sedi- 
mentary rocks dip away from the Butte in all directions ; the 
harder strata form more or less continuous ridges separated by 
depressions carved in the shales, and encircling the mountain. 
Near the igneous rocks the beds are much hardened and altered 
by contact metamorphism. 
The dike rock described herein* occurs as a broken wall, 
traversing the horizontal sandstones and clays ten miles north 
of the summit of the East Butte. Its course is east and west. 
The West Butte is the largest of the three, and rises toa 
height of 6,500 feet. It forms a mountainous area with num- 
erous round-topped peaks and ridges separated by deep, pre- 
cipitous valleys. The highest point is a large, blunt-topped 
summit with vertical rocky cliffs forming the eastern face. 
The sedimentary strata, dipping away from the peaks of igneous 
rock, show considerable contact metamorphism, and large areas 
of these altered rocks occur in the central portion of the 
mountain. 
The structure here described corresponds closely with that — 
observed in the mountain groups to the south of the Sweet 
Grass Hills. The Moccasin mountains, two isolated mountain 
masses, rising above the plains south of the Missouri, near the 
Judith mountains, consist of igneous cores whose rocks closely 
resemble those of this region, and are clearly laccolitic in 
character. Similar laccolites occur in the northeastern part of 
the Little Belt range, and the eruptive rocks of the outlying 
mountain groups, the Bear Paw, Little Rocky and Judith 
ranges, are also, in part at least, laccolitic. At most of the 
localities, however, the Paleozoic rocks are exposed, the intru- 
sion having occurred in the shales forming the base of the 
Paleozoic series, whereas the Sweet Grass rocks are all of late 
Cretaceous age. 
The igneous rocks here described are of similar types and cor- 
respond closely to those forming the laccolitic mountains just 
mentioned. 
* See description by Dr. Dawson, loc. cit., p. 45. 
