112 THE PENOKEE lEON-BEAEmG SERIES. 



already described, except that horubleiide takes fhe place of biotite, so that 

 this mineral and Qhlorite are the chief constituents aside from the quartz and 

 feldspar. The hoi-nblende of the ordinary variety is found in large well 

 develo})ed blades and crystals. It is at times included within the feldspar, 

 but iu general it is between different individuals of that mineral, although 

 even here the relations are such as to show that tlie feldspar has acconuno- 

 dated itself to the hornblende. This nuist mean that if the hornblende is 

 an original mineral it was the first one to crystallize. Its pleocliroism is: C, 

 dark bluish green ; ll, dark yellowish green ; a, light yellow ; absorption, jj 

 about equal to b > n. The chlorite of the rocks, where it is found instead 

 of the hornblende, bears the same relation to the feldspar that the horn- 

 blende does. In some of the sections the chlorite is in part secondary to 

 hondjlende and it may all be of this derivation. 



Constituting an exception to the granites just described is an exposure 

 of microgranite found in the NE. ^ of Sec. 27, T. 47 N., R. 47 W., Micliigan. 

 Macroscopically it has an aphauitic background which contains simple and 

 complex areas of varying sizes, consisting of coarse individuals of feldspar, 

 a dai-k colored mineral, and quartz. In other words the background in- 

 cludes many areas which have a true granitic texture. These complex areas 

 of greatly varying sizes are included in the most iiTegular fashion. More 

 abundant than these are single porphyritic crystals. In thin section the 

 backgi'ound is very finegrained, and apparently consists of intricately inter- 

 locking quartz and feldspar. As indicated by the hand specimen it contains 

 numerous roundish areas consisting of large individuals of quartz and feld- 

 spar, with quite a quantity of chlorite, each mineral occurring separately 

 and togetlier in complex, intricately interlocking areas. Quite a number of 

 the single individuals of (piartz show plainly an interruj)ted growth, which 

 gives each grain an appearance like that of enlarged quartzes met with in 

 fragmental rocks. Here this doubtless means that the cores belong to a 

 first generation and the enlargements to a second generation of crystals. 

 The porphyritic feldspars include orthoclase, microcline, and plagioclase. 

 This rock differs from any which I remember to have seen. In most cases 

 in which crystallization has thus occuiTed in two generations only detached 

 porphyritic crystals are contained in a finer grained matrix. Here, how- 



