THE ARCH^AN ROCKS. 



509 



about the Devil's Lake valley. For a further idea of the structure of thia vaUey refer- 

 ence is made to the sections of Plates XIX and XX. 



The rook in the vicinity of Devil's Lake, omittuig reference now to the SUurian con- 

 glomerate and sandstones, is nearly altogether the typical quartzite of the region, as 

 above described. It generally shows some shade of red. On the weathered surfaces of 

 some of the large fallen masses in the edge of the lake, a distinct tendency to a granular 

 texture is perceptible, whilst a fresh surface shows generally no traces of it. Fine lines 

 of lamination are neai-ly everywhere to be seen, and are generally quite strikingly 

 marked, but there is never any structure parallel to them. They are nearly always bent 

 into shai-p angles, or curved and contorted, presenting often the irregularities seen in 

 the bedding of sandstone. "Whilst many of the bendings in these Unes may be due to 

 original irregularities of deposition, or to contortion at the time of disturbance and alter- 

 ation, there are surfaces where they present such a peculiar knotty and concentric ap- 

 pearance as strongly to suggest a concretionary origin. A portion about 3 feet square 

 of such a surface is figured in Fig. 24. The hnes are altematingly light and dark red. 



Fig. 24. 



ScEFAcn OP Quartzite showing oubvbd Laminatiok. 



In a few places white quartz veins with geodio quartz crystals are seen, but. these do not 

 characterize any considerable portion of the rock. All about the Devil's Lake valley the 

 bedding of the quartzite is quite distinct, and is made apparent by the existence of large 

 dip surfaces, often beautifully ripple-marked. At the northern ends of both east and 

 west bluffs of the lake many such surfaces occur. Others are seen on the sides of the 

 railroad track about midway the length of the lake. These aU give an inclination to the 

 north of 15° to '&' , the higher figure being seldom reached, and a strike of N. 80° E. 

 The quartzite layers are many feet in thickness, shovnng no internal structure whatever 

 parallel to the general dip direction, but being affected everywhere by the curved and 

 bent lamination alluded to. Between the quartzite beds occur layers of greasy quartz- 

 slate, usually but a few inches in width. Such a layer is well exposed on the side of the 

 railroad track on the east side of the lake, the laminEe dipping N. 37°, or transverse to 

 the bedding planes. The slate is quartzite, Uke that of the surrounding beds, but is 

 penetrated by a soft, greasy mineral, and affected by slaty cleavage. As the cleavage 

 planes of the slate approach the surface of the adjoining quartzite, they curve towards 

 and penetrate it to a short distance, as indicated in Fig. 25. Large suri'aces of quartzite, 



