95(5 THE GEOLOGY OP MINNESOTA. 



[Diallage. Hypersthene. 



Diallage. The diallagic cleavage of augite is common in the diabase and in the 

 gabbro of the Keweenawan, as well as in the muscovadyte. In the diabase it is 

 seen in Nos. 115, 222, 1605, 1287. In the gabbros in Nos. 1C, 698, 985, 1136, 1137, 

 1287, 1678, 1749. In the muscovadyte it is conspicuous in Nos. 122, 1287, 2199, 2201. 

 Diallage has not been observed in the Archean rocks. 



From all that has been observed, the diallagic characters of augite appear to 

 have an early date. Far from being due to secondary causes, and hence a secondary 

 feature of the pyroxenes of the Keweenawan, the characteristic lamellation appears 

 rather to be one of its primordial characters. It is most frequently seen in those 

 pyroxenes that antedated, or were coeval with, the plagioclase and olivine (Nos. 1C, 

 1287). It is in the muscovadyte that it is intertwined in a lamellar succession with 

 enstatite (Nos. 1340, 2199) and with hypersthene (No. 2202). 



There is, however, a diallagic structure which results from late alteration of 

 augite, and in some instances it has been accepted by petrographers as the true 

 structure of diallage (No. 300), but it should be kept distinct. There is no doubt 

 that this confusion of two structures under one name has been the cause of much 

 difference of opinion as to the nature, origin and date of the mineral diallage. That 

 diallagic lamellation which is of later date and is attributable to natural decay from 

 weathering or other influences is fine and fibrous, is parallel to the base of the 

 augite crystal, and it destroys the orientation of the augite which is affected by it. 

 It is described and illustrated by Wadsworth in Bulletin ii of the Minnesota Survey, 

 plate VII, figure 1, and page 80. But that lamellation parallel to 100, which is the 

 structure that characterizes the oldest diallage of the gabbro and of the muscova- 

 dyte, does not destroy the orientation of the augite. Non-diallagic, ophitic augite 

 often exists in the same rock with true diallage (No. 222), and true diallage is some- 

 times also ophitic (Nos. 115,8470). 



There seems to be a fundamental difference between diallage and diopside, viz.: 

 The true diallagic lamellation 100 perpendicular to the optic plane is original and 

 primary, and exists in the gabbro the result of refusion of the (usually) clastic green- 

 stones, but that of diopside (010) parallel to the optic plane is a character 

 of the pyroxene developed later, as in gneiss and crystalline schists, as well as 

 in some of the so-called augite granites, both of the Archean and of the Kewee- 

 nawan. 



Hypersthene. This is the most common of the orthorhombic pyroxenes. It is, 

 however, practically restricted to the muscovadyte series, i. e., to the zone involving 

 the transition from the old Keewatin elastics to the gabbro of the Keweenawan, 

 whether considered genetically or geographically; but it also continues slightly 

 beyond the transition, on the gabbro side of that zone, and thus gives name to a 



