STRUCTURAL AND STRATIGRAPHICAL RELATIONS . 453 



zonally built with pale green centers and darker green rims, both, how- 

 ever, with large extinction angles. Eare hornblendes, five or six times as 

 long as broad and of a beautiful green color, are likewise present. There 

 are no feldspar phenocrj^sts, but in the groundmass the little rods, once 

 twinned for the most part and with nearly parallel extinction, are the 

 chief components. There is much magnetite, and frequent and fairly 

 large, rudely prismatic, apatites are scattered among the small compo- 

 nents. There are evidences that some rather large component has been 

 resorbed, leaving magnetite skeletons, and there are rare residues of cal- 

 cite and feebly refracting serpentine or chlorite, representing an original, 

 now hopelessly altered. The rock may be best described as a minette, 

 but it is not so rich in biotite as some others in and north of East Butte. 

 The most remarkable feature of sill 39 is furnished by the innumerable 

 inclusions, up to 2 feet, in diameter, of Precambrian rocks. They can 

 be seen obscurely in figure 2 of plate 10, and in places constitute a third 

 or a quarter of the mass. As they are all resistant rocks, they show 

 slight, if any, effects of absorption or digestion. They must represent 

 either fragments along a fissure or zone of parallel Assuring through 

 which the sill drove its way like a wedge, or else were stoped ofE as the 

 magma ate its way upward. Among the fragments the following varie- 

 ties were recorded: granite gneiss, hornblende gneiss, granite, quartzite, 

 bits of pegmatite, garnetiferous granite gneiss, and mica schist. There 

 were also rounded inclusions, which, in the one case studied, consisted 

 chiefly of green, granular diopside at the center, with a black border of 

 hornblende. The total diameter was -to millimeters and the border 10 

 millimeters across. This inclusion was probably once limestone. In 

 another of similar nature garnets were noted. 



EAST BUTTE 



In areal uplift East Butte is the largest of the three, and brings to the 

 surface the Madison limestone, the oldest of the sedimentary strata ex- 

 posed in the Hills. The limestone is domed up by the large southern 

 laccolith, with its two peaks at E. 1 and E. 2, figure 5, and is penetrated 

 by the more irregular bodies, E. 3, E. 4, and E. (5. Smaller outlying 

 laccolithic masses appear around the main intrusions at higher strati- 

 graphic horizons. The flanks of the uplift present a maze of sheets anrl 

 dikes. Of especial structural interest is a veritable plug, 10 feet in diam- 

 eter, which pierces the Madison limestone at specimen 125. 



East Butte contains the rocks of greatest petrographic intei-est. Tiiere 

 appears to be a progressive change in chemical composition in the out- 

 l)reaks of the three buttes from west to east. 



