Platk X.— WEWE SLA.TE AND SIAMO SLATE. 



Fig. 1. Recomposed rock, resembling granite, from tiie Wewe slate near the center of sec. 22, T. 47 N., 

 R. 26 W. (Atlas Sheet XXXV). The specimen is taken from near the liase of the forma- 

 tion. The underlying Archean rock is granite. The discrete mineral particles of the 

 granite form the detritus of the figures. These, when cemented, produced a rock very 

 similar in appearance to a gneiss. Indeed, in the hand specimen it is almost impossible to 

 discriminate this rock from true gueissoid granite of the Archean, but in thin section the 

 fragment.al character of the specimen figured is in strong contrast Tvith the completely 

 crystalline character of the gueissoid granite. The recomposed rock has been somewhat 

 broken by dynamic action, and along the cracks veins h.Tve formed. Natural size. 



Fig. 2. Ferruginous Siamo slate, showing oxerthrust fault, from the top of the formation in sec. 35, 

 T. 48 N., R. 27 W. (Atlas Sheet XXVII). The specimen is cut diagonally across the bedding, 

 so that the layers appear to be wider than they really are. The finely laminated, greenish- 

 gray portion is typical of the less altered varieties of the Siamo slate. At the bottom and 

 top of the formation this material is frequently iuterlaminated with iron-stained layers, 

 and the figure shows a typical case of this kind. Though the ferruginous bands approxi- 

 mately follow the bedding, they cut across it in such a way as to show that, while the 

 percolating waters were controlled in a large way by the bedding, to some extent they 

 went across it. A study of the thin section shows that the ferruginous layers usually 

 develop where there was originally siderite. In one of the gray bands an overthrust 

 fault is beautifully shown. This has sharply broken the harder, more siliceous layers 

 and has carried with it the weaker layers between the harder ones. However, both above 

 aud below, the fault passes into a flexure. The specimen was evidently in the zone of 

 combined fracture and flowage, the readjustment of the harder layers being by fracture 

 and that of the softer layers by flowage. This fault, although on a minute scale, illustrates 

 perfectly how a major fault may disapiJear below by passing into a flexure. Natural size. 

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