182 THE MARQUETTE IROX-BEAHING DISTRICT. 



THE ACID DIKES. 



The acid dikes cuttin<i- the rocks of the Northern Complex are not so 

 nnmerons as are the basic ones, but their variety is greater. They include 

 coarse granites and granite-porphyries, iine-grained granites, ajslites, quartz- 

 porphvries, and the aplitic form of qnartz-diorite known as malchite. 



No descriptions of the coarse granites and granite-porphyries are 

 necessary. They are apojihyses of the great granite masses north of the 

 schists and do not differ in character from these. They are of the same 

 age as the gneissoid granites, an<I hence are older than the Aarious aplitic 

 and diabasic dikes that intrude the granite. 



The remaining acid dikes are usually of inconsiderable size when 

 com]3areil with the great basic dikes that traverse the schists and granites. 

 They are als(j of various ages. Some are foliated and others are massive. 

 The former were intruded before the last effects of pressure had been 

 imjDressed upon the schists, and the latter long after the schists were made, 

 for they intersect even some of the massive diabase dikes. None of them, 

 however, intersect any of the members of the Algonkian series. The color 

 of the ilikes A'aries from ])inkisli-gray, through pink, to a l)rig]it red. Their 

 material is always compact, and except when here and there it is microlitic 

 it is also very fine grained. Many of these dikes are fine-grained granites 

 with no ])eculiar features. Their feldspar is usually red or pink, and their 

 principal bisilicate is a chloritized biotite. Others partake more of the 

 aplitic character. Their feldspathic com])onent possesses more or less per- 

 fectl\- ([iiadrangular cross-sections, and their quartzes circular ones. 



Both the granites and the aplites are altered. Their orthoclase is kao- 

 linized and their biotite is so completely changed to chlorite that it is 

 often difiicult to determine what were the original components. Epidote 

 is not infrecpienth- an alteration product of the jjlagioclase |)resent, and it is 

 apparently often a result of the decomposition of biotite. 



A i\'\y dikes of a fine-grained, dark-gray rock are of interest, since they 

 represent the aplitic- form of quartz-mica-diorite, named malchite by Osaun.' 

 The best specimen of this rock came from a dike 600 steps north of the 



'Mittheil. Gross. Bad. geol. Landesanstalt, Vol. II, p. 380. Cf. also Microscopic study of some 

 Michigan rocks, by H. B. Patton : Rept. State Bo.ard Geol. Surv. Michigan 1893, pp. 184-186. 



