164 Spttrr — Stratigraphic Position of the Thomson Slates. 



The rock is in most places of evident sedimentary origin. 

 It varies from a fine-grained slate, black, gray, or dark-green in 

 color, to a siliceous slate or graywacke. Often small pebbles 

 of detrital material, especially of quartz, may be found. The 

 sedimentary structure is usually indicated by no parallel part- 

 ing, but by the alternation of coarser and liner layers. An 

 observation of these alternations shows that there is no con- 

 stant dip and strike, but that the rocks have been much folded 

 and crumpled. There are, however, persistent and continuous 

 systems of cleavage. There are two principal sets of cleavage 

 planes, one running from !N". 80° E. to E. and W., and the 

 second cutting the first at an angle of about 45°. The first 

 cleavage is best developed, and gives the appearance of strati- 

 fication to the formation. "Where it is most strongly marked, 

 there results a very perfect division of the rock into thin 

 sheets; where both are well developed, the rock is cut up into 

 fine rhombohedral blocks. In some of the coarser rocks, such 

 as the graywacke, the structure is rather schistose than slaty. 

 It is to be noted that neither of the cleavages corresponds to 

 the bedding, or has any constant relation to it. 



In places, where the rocks are cut by basic dikes, a third 

 cleavage has been induced, extending for a few yards on either 

 side of the dike, and running strictly parallel to its walls. 

 This third cleavage cuts the other two, and near the dike 

 almost obliterates them. The predominant cleavage or schis- 

 tose structure, which runs a little north of east, has been very 

 commonly described as stratification by previous writers. The 

 minor parting, however, has been very generally recognized as 

 a cleavage. Throughout the rock there are numerous irregu- 

 lar veins, chiefly of quartz. 



This series was traced by the writer as far north as T. 51-19,. 

 near the St. Louis River. Here several specimens were taken 

 from an outcrop (about ~N.W. J S.W. J, Sec. 27). One phase 

 is a dark green siliceous slate or quartzite ; another is a fine- 

 grained slate ; a third is a silvery sericitic schist or slate. In 

 the two last-named rocks what appears to be the original strati- 

 fication may generally be distinguished ; and in the specimens 

 taken this structure runs nearly at right angles with the cleav- 

 age. It is not so plain, however, as ten miles further south. 

 There was found at this place only one cleavage or schis- 

 tosity (for in different zones it appeared as each) in the rock. 

 This corresponded in general with the main cleavage at 

 Cloquet, with a steep hade, and a trend between N. 60° E. and 

 K 80° E. 



