414 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. . 



elements which could not influence its results. The very large amount 

 of P2O5 is noticeable, and proves what the microscope reveals, the large 

 amount of apatite present. The amount was fixed by two closely agree- 

 ing determinations. 



Analysis of iShonJcinite.* 



SiO^ 46.73 



TiO^ 78 



AlA 10.05 



FeaOs 3.53 



FeO 8.20 



MnO 28 



MgO 9.68 



CaO 13.22 



Na,0 1.81 



K2O , 3.76 



H2O 1.24 



P2O5 1.51 



CI 18 



100.97 

 = C1 04 



Total 100.93 



To be noted here is the low silica and very high magnesia, iron and 

 lime. It is evident that although the feldspar raises the silica percentage 

 it is not in sufficient amount to counteract the olivine, iron ore, biotite, 

 apatite and other minerals which tend to lower it. The water comes in 

 part from zeolites. 



Structure and Classification. — The minerals in the order of their crys- 

 tallization are, first, apatite, then iron ore, olivine, biotite and augite. 

 The period of the last two overlaps. Then followed the feldspathic com- 

 ponents, whose succession is quite doubtful as regarding one another, 

 except that on the whole the albite-anorthoclase group appears to be 

 among the earliest. 



These minerals lie unoriented, forming a holocrystalline, rather coarse 

 granular hypidiomorpic structure. It resembles in many respects the 

 coarser grained theralites of the Crazy mountains ; in others certain coarse- 

 grained dolerites. The structure is illustrated on the next page by 

 figure 8, which indicates ' also the prominent position the augite plays 

 in the composition of the rock. 



From what has been given in the foregoing description it is evident 



*By L. v. Pirsson. 



