GABBdO AXD ISrORITE INTEUSIVES. 249 



pyrites are also found in the altered rocks. These secondary products are 

 easily distinguished from the original minerals by the total absence in them 

 of wavy extinction. The effect of the crushing upon the texture has been 

 to render it more or less schistose, the resulting rock resembling in its 

 mineralogical character, and in texture, an ordinary hornblende-gneiss, or, 

 when quartz is present in but small quantity, a plagioclase amphibolite. 



RELATIVE AGES OF GABBROS. 



An attempt to determine the relations of the varieties described resulted 

 in the establishment of the following order of inti-usion: The typical 

 coarse-grained hornblende-gabbro was the first formed, and seems to be the 

 most widely distributed. This was then intruded by dikes of gabbro, which 

 contain both monoclinic pyroxene and hornblende in association and rep- 

 resent a gradation to the normal gabbro. These hornblende-gabbros and 

 normal gabbros were then cut by the dikes of bronzite-norite and bronzite- 

 norite-porphyry. 



PEEIDOTITBS. 



Until recently all of the ultrabasic rocks were included by most 

 petrographers under the family name of peridotite. In the last edition of liis 

 Mikroskopische Physiographic, Rosenbusch has separated these rocks into 

 the plutonic rocks and the volcanic rocks. The tei'm " peridotite " is 

 restricted to the first, and the last are called the "jDicrites." The charac- 

 teristic differences between these two types are very clearly shown in the 

 specimens from the Crystal Falls district which I haA'^e had the oppor- 

 tunity of studying. The rocks here described agree with Rosenbusch's 

 narrower definition of the peridotites.-^ 



DISTRIBUTION, EXPOSURES, AND RELATIONS. 



The peridotite dikes were all found near the Michigamme River, in 

 sees. 29 and 22, T. 42 N., R. 31 W. Typical wehrlite with very little green 

 amphibole is found on the east bank of the Michigamme river, near the 

 center of sec. 22, T. 42 N., R. 31 W. It shows no relations to the other 

 rocks. The amphibole-peridotite, exhibiting marked variations to wehrlite 

 and olivine-gabbro, forms an outcrop on the east bank of the Michigamme 

 River in the NE. ^, sec. 29, T. 42 N., R. 31 W. This dike cuts the gabbro. 



' Op. cit., p. 343. 



