1888.] 495 [Crosby. 



tion to a finely laminated schist which, considered by itself, shows 

 but slight traces of its conglomerate origin. 



The pebbles of the conglomerate are chiefly vitreous quartz, 

 quartzite and siliceous schists, derived, apparently, from the older 

 Archaean series ; and the quartzite pebbles largely predominate. 

 The pebbles of vitreous quartz appear to have very generally es- 

 caped distortion ; and I have been unable to discover any evidence, 

 either in the Black Hills or Massachusetts, that the individual 

 grains of quartz composing the quartzite pebbles are flattened or 

 lengthened ; but the deformation of these pebbles appears to be 

 accomplished by the slipping of the grains of quartz over each 

 other. We may conceive that under the enormous pressure the 

 grains in the quartzite behave somewhat as in loose sand, and ex- 

 perience differential movements or flowing. 



A careful examination of the conglomerate in all its aspects 

 shows that it has suffered powerful compression in two directions, 

 which, in the Black Hills, are usually approximately horizontal, 

 the pressure perpendicular to the bedding usually predominating. 

 The pebbles and paste have thus been extended or squeezed out 

 mainly in the third direction, which is usually approximately ver- 

 tical. The general result of this linear deformation of the peb- 

 bles is to give the rock a structure which is rather more properly 

 columnar than schistose ; and in tracing this mode of metamor- 

 phism from the coarser to the finer grained rocks of this sediment- 

 ary series, it is found to afford an adequate explanation of a struct- 

 ural feature of the Archaean quartzites which has greatly puzzled 

 many observers in the Black Hills. 



The Archaean quartzites are very generally characterized by a 

 curious rectilinear and vertical streaking or columnar structure,, 

 which suggests bedding lines at the first glance, but on closer ex- 

 amination often bears some resemblance to the Scolithus borings 

 of the Potsdam quartzite or the stylolite structure often seen in 

 massive limestones. A careful study shows that the streaking 

 usually affects the entire body of the rock, although it has often 

 been locally emphasized through the development of hydromica 

 and other secondary minerals. These marks are doubtless some- 

 what akin to stylolites in their origin. Although the component 

 grains of the quartzite have not probably suffered extensive defor- 

 mation, it is impossible to resist the conclusion that the rock as 

 a whole must have yielded, through the gliding of its grains over 



