Weed and Pirsson — Bearpaw Mountains, Montana. 297 



noticeable difference is that which is so striking megascopically, 

 the lack of large phenocrysts ; a few indeed do occur and they 

 are similar to those previously described but are comparatively 

 very small. 



The most interesting feature then of this contact form is 

 the sharp contrast between its almost total lack of feldspar 

 phenocrysts, with the main type in which they occur in such 

 great size and profusion. It shows clearly that these pheno- 

 crysts are not of intratelluric formation, since in that case 

 they should occur in both forms alike, but that on the con- 

 trary they have developed by far the greatest part of their 

 bulk during the final period of consolidation, and thus confirm 

 the ideas concerning their formation previously expressed. 

 Such facts are of great importance to a proper understanding 

 of the porphyritic structure, and the present case is a confirma- 

 tion of the observations of Cross,* who has thoroughly dis- 

 cussed the bearing of such facts on theoretical petrology; 



Cleveland Butte. — The summits south of Cleveland are 

 probably formed mainly of the effusive basaltic rocks, as is 

 shown by the character of the stream drift from their slopes, 

 but massive rocks also occur. The summit forming the eastern 

 end of the range of hills south of Cleveland shows an intrusive 

 rock that differs from those described. 



The rock is a trachyte of a medium gray color, with a some- 

 what waxy greenish tone. It is thickly spotted with small 

 prisms of black hornblende of varying size; they are rather 

 slender and average 1 to 3 mm in length ; occasionally they are 

 much larger, and exceptional individuals a centimeter or so 

 long are sometimes seen ; quite rarely irregular or stellate 

 aggregations of them occur. Feldspar phenocrysts are rare ; 

 they are 1 or 2 mm in diameter. The dense, greenish-gray, feld- 

 spathic ground-mass and the hornblendes give the rock its 

 prevailing character. It weathers with a brownish crust. 



In the section the hornblende is found to be generally idio- 

 morphic but • always more or less resorbed and coated with 

 borders of black opacite grains ; in some cases this process has 

 gone so far that nothing is left of the original crystal but a 

 mass consisting of black ore grains, shreds of brown biotite 

 and granules of diopside mixed with fibres and masses of the 

 original hornblende. The mineral is of the usual olive-green 

 character with pleochroism into yellowish tones ; it has the 

 small angle of extinction and rather low birefraction so com- 

 monly found in rocks of this class. 



The other prominent ferro-magnesian mineral is a pale-green 

 diopside, which is pretty freely sprinkled through the ground- 



* Laccolitic Mountain Groups, etc., 14th Ann. Rep. U. S. Geol. Surv. for 1892- 

 1893, p. 228 et seq. Washington, 1895. 



