Rock from the Highwood Mountains of Montana. 317 



Upon examination the dark constituents may be distinguished 

 as chiefly a greenish black augite in columnar masses and aggre- 

 gates which are never idiomorphic, together with an occasional 

 speck of a bronzy brown biotite of ill-defined outline or a grain 

 of a deep yellow olivine. Filling the interspaces between these 

 dark minerals in formless masses is a very pale greenish gray 

 substance which is leucite. The average size of. crystal grain 

 varies from 2 to 5"™, so that the rock is of quite coarse granu- 

 lar structure, and it resembles most strikingly in fact many 

 coarse-grained gabbros. 



Microscopic characters. — The thin section under the micro- 

 scope shows the minerals present to be ajjatite, iron ore., 

 olivine^ augite^ hiotite, leucite^ and some zeolitic products. 



The apatite and iron ore., which are present rather rarely in 

 moderate-sized grains, show nothing of especial interest beyond 

 that they are found enclosed in the other minerals, and the bio- 

 tite frequently encloses the iron ore. 



The olivine is extremely fresh, unaltered in any way, and 

 resembles the olivine of fresh gabbros. It contains great 

 numbers of very fine glass and iron ore enclosures. It never 

 shows any crystal faces, but is in rounded, formless, anhedral 

 grains which are frequently enclosed in biotite and augite. 



The augite is of a pale green color with a tone of brown ; it 

 is very fresh and clear, contains enclosures of ore and specks 

 of biotite and is entirely allotriomorphic, though the orienta- 

 tion of the ore grains is at times zonal, thus indicating crystal 

 planes. It has an excellent cleavage and twinning bands pass 

 through it in places; it does not show any pleochroism. 



The biotite is strongly pleochroic between a deep umber 

 brown and a pale yellow brown ; it is also entirely allotrio- 

 morphic though apt to surround the other minerals in bands, 

 especially the olivine and iron ore. It is particularly character- 

 istic in such cases that it then passes from brown into an olivine 

 green variety which has a mottled, somewhat stringy, fibrous 

 appearance. It appears in these cases as if the brown variety 

 had suffered from some magmatic process ; it does not seem to 

 be due to any ordinary process of weathering. 



Leucite. — The leucite appears also like the other minerals in 

 formless masses filling the interspaces between other minerals. 

 It is perfectly clear and free from all inclusions, except now 

 and then a grain of the ferromagnesian minerals. Between 

 crossed nicols it shows most beautifully the cross-banded twin- 

 ning structure so characteristic of leucite. It is in general 

 perfectly clear, limpid and fresh, though in some areas, in deli- 

 cate fringes along cracks and on the borders of grains, a low 

 birefraction shows that processes of zeolitization have com- 

 menced. This will be described more in detail later. 



