BASIC INTEU8IVES. 199 



veiy similar to the basalts have already been described at length under the 

 Hemlock volcanics, and since they are found in comparatively few dikes 

 they will be passed over with very brief mention. 



METADOLERITE.' 

 G-EOGEAPHICAL BISTEIBUTION. 



The dolerites of the Crystal Falls district for the most part form high 

 ridges extending in a northwest-southeast direction. Their principal occur- 

 rence is in the area immediately north, northeast, and east of Crystal Falls. 

 Beginning in seCs. 32 and 33, T. 43 N., R. 31 W., there extends a great 

 intrusive mass, varying from a mile to a mile and a half in width, due north 

 to sec. 6, T. 43 N., R. 31 W. There it bends to the northwest, and ends in 

 sec. 3, T. 43 N., R. 32 W. The northwestern extension of this mass is much 

 narrower, never exceeding a half mile, and at many places it is only a few 

 hundred yards in width. In the northern part of T. 42 N., R. 31 W., are a 

 number of knobs which are evidently connected below with these lai'ge 

 masses, although the exposures are discontinuous. 



A large dike begins in sec. 1, T. 42 N., R. 32 W., and extends for about 

 5 miles to the northwest into sec. 19, T. 44 N., R. 32 W. This averages 

 about one-eighth of a mile in width, though in places it is three-fourths of 

 a mile wide. Another dike begins in sec. 28, T. 44 N., R. 32 W., and runs 

 for 3J miles to the northwest into sec. 18, T. 44 N., R. 32 W., and, like the 

 above, is narrow, being only about one-eighth of a mile in average width. 

 A narrow dike less than one-eighth of a mile in width extends in a high 

 ridge from in sec. 16, T. 43 N., R. 32 W., to the northwest for 3 miles and 

 ends in sec. 7 of the same township and range. Numerous isolated knobs 

 occur in T. 44 N., R. 32 W. A small boss is in sec. 24, T. 46 N., R. 33 W., 

 and another at 1,600 paces N., 1,000 W., sec. 19, T. 47 N., R. 33 W. 



PETROGRAPHICAL CHARACTERS. 



Macroscopicai. — Tlic dolcrites Vary in color from greenish to dark olive- 

 green and almost black. The weathered surface is usually of a very light 

 color, rather a light gray, with frequently a reddish tinge. The texture is 



'I use the name "dolerite" here merely to indicate the macrostructural difference between the 

 rocks included under it and the fine-grained and aphanitic rocks of the same composition included 

 under the basalts. It is also extended to include jia^cti as well as neo eruptives. As nearly all, if not 

 all, the paleodolerites have undergone great alteration, the prefix meta — indicating alteration without 

 reference to any specific kind — is very generally applicable. 



