946 THE GEOLOGY OF MINNESOTA. 



[Epiclote. Zoisite. Zeolites. 



Epidote. This is a very common but not very abundant mineral. Its earliest 

 appearance is as small grains or groups of grains in the Lower Keewatin greenstones; 

 where it is plainly the result of alteration of the feldspars in presence of iron. It is 

 essentially a silicate of alumina, lime and iron. From this primary source (and by 

 analogous production in later rocks) it is disseminated amongst the later rocks that 

 are dependent on the oldest basic crust for their essential characters. It is also 

 common in scattered grains in the more acid of the schists of the Archeau, which 

 also probably received the elements of which it is composed from the basic rocks of 

 the earlier Archeau. 



It is a rather common mineral in the Archean granites, quartz-porphyries (?) 

 (No. 914), and gneisses (Nos. 435, 994, 995), and especially in the " intermediate " series 

 represented by the diorytes and by the amphibolytes (Nos. 401, 403). It here forms 

 isolated crystalline grains, and its date appears to be as early as that of any of the 

 constituent minerals of those rocks. Whether it was incorporated as epidote into 

 the original clastic materials of which these rocks are believed to be primarily com- 

 posed, or has originated through metamorphic recrystallization, it is difficult to affirm 

 with present data, but from its general absence from the gray wackes and other non- 

 crystalline detritals of the Archean (see, however, No. 488), it appears to have been 

 in the main the product of recrystallization under the action of metamorphic forces. 

 It is therefore probable that, in the Archean, epidote is one of the distinctive meta- 

 morphic minerals. 



It does not, however, depend wholly on metamorphism, for it is found in the 

 Keweenawan basic rocks, where it seems to have been produced by ordinary weath- 

 ing, or by the action of heated solutions on the lavas during the period of cooling, 

 after congealation (Nos. 567, 569, 697). 



In a single instance (No. 842) has it been found abundant enough to control the 

 nomenclature of the rock (epidosyte). It is there associated with albite. 



Zoisite. Zoisite is allied to epidote, but is practically free from iron, and it is 

 probably less the product of metamorphism. It is rather the direct product of simple 

 feldspathic alteration produced by solfataric exhalations or by heated solutions (Nos. 

 868, 872, 922, 1802). Accompanied by more or less of chlorite, mica and new feld- 

 spars, it constitutes very largely the impurities that pass under the general name 

 saussurite. A saussuritized feldspar may be, and is frequently, embraced in the 

 detrital rocks. Under such conditions, on the advent of metamorphism zoisite 

 seems to maintain its identity in the resultant schist or dioryte (Nos. 861, 881). 



Zeolites, of which a considerable number of species have been identified, are 

 likewise the result of alteration of eruptive basic rocks or of their debris. The index 

 may be consulted for reference to the important discussions. 



