254 THE (JKYSTAL FALLS IROM-BEAliliSfG DISTRICT. 



they never include each other. They are both, however, included in the 

 hornblende, which with the biotite forms, as it were, the mesostasis. Biotite 

 is present in this specimen in very small ([uantity, and is essentially the 

 same kind as that above described (p. 252), except that it shows a trifle 

 hio-her absorption parallel to the cleavage and becomes a yellowish-red. 

 The rock is very fresh and shows scarcely any traces of alteration. This is 

 partly due to the erosive action of the Michigamme River having removed 

 the weathered crust, thus making fresh specimens obtainable. 



This rock, from the description just given, would be classified as an 

 amphibole-peridotite, with accessory diallage, bronzite, and biotite. It 

 approaches Williams's cortlandtite. In some specimens the biotite is pres- 

 ent in very large quantity, though hardly in sufficient quantity to warrant 

 the designation of any of the rocks as biotite-peridotite. 



GRADATIONS OF AMPHIBOLE-PERIDOTITB TO WEHRLITE AKD OLIVINE-GABBBO. 



There were taken from the same exposure whence the above-described 

 amphibole-peridotite came some specimens which raacroscopically can not 

 be distinguished from those of the amphibole-peridotite except in that they 

 are a trifle finer grained. Examined under the microscope, however, we 

 find differences. In some the hornblende is very much reduced in quantit}-, 

 and varies from the brown kind just described to a light-greenish color, the 

 two being in optical continuity, and the augite and oHvine are increased in 

 quantity. These are good types of a welirlite. In some of the wehrlite.s 

 there is a variable percentage of feldspar. In certain cases it reaches an 

 amount which would almost warrant the classing of the rock as an olivine - 

 gabbro. Patton described a rock from the same outcrop in which the horn- 

 blende still predominated, but in which there was also a certain amount of 

 plagioclase.^ He called it a hornblende-picrite.^ According to the ter- 

 minology here used, if the plagioclase is to be neglected, it would be an 

 amphibole-peridotite. 



The thin sections of the feldspathic phase of this rock seem to show 

 that it approaches more closely to a gabbro — that is, to be more feld- 

 spathic than the one described by Patton. They certainly contain far less 

 hornblende than his, judging from his description, and more feldspar. The 



' Mikroscopische PhysiograpUie, by H. Eoseubuacb ; 3(1 ed., Stuttgart, Vol. II, 1896, p. 352. 

 -Op. cit., p. 186. 



