KOCKS OF SAINT LOUIS RIVER DISTRICT 849 



alteration phenomena the segregation into nests of carbonates of iron, 

 calcium, and magnesium was effected. 



STRUCTURE AND DISTRIBUTION OF THE CONCRETIONS 



Ordinarily these concretions are arranged in bands. As the rocks are 

 eroded or cut away by human agency, the concretions stand in rows 

 one above the other along the line of bedding. They are so compressed 

 that they stand in the direction of the slaty cleavage, so far as individual 

 position goes (see plate 31, figures 1 and 2) ; hence they give evidence 

 both of original position and effect of pressure. They are undoubtedly 

 of secondary origin ; they consist chiefly of iron carbonate in chemical 

 composition ; they are quite well defined as against the mass of rock 

 enclosing them, and weather with unusual facility and rapidity. 



QUARTZ VEINS 



There is a large number of quartz veins. The largest one noted is a 

 segregation which stands up in the river bed where the carriage bridge 

 from Carlton to Thomson crosses the Saint Louis river. It carries much 

 of the slate and exhibits a heterogeneous quantity of quartz, and smaller 

 quantities of pyrite, chalcopyrite, and traces of associated sulphides. 



The smaller quartz veins scattered throughout the rocks diminish to 

 paper thinness. They are milky white, harder than the slate and quartz- 

 ite through which they course, and are remarkably free from accessory 

 minerals. The closest scrutiny of explorers has failed to find more than 

 traces of gold and silver. 



DIABASE DIKES 



The somewhat frequent dikes are objects of interest. The greatest 

 width noted is 50 feet in one crossing the slates and quartzites one mile 

 southeast of Carlton on the Northern Pacific railroad. The rock is 

 diabase, extremely finely textured along the contact zones, and mediumly 

 crystalline in its central part. At 50 paces east of railway bridge between 

 Thomson and Cloquet is a 3-foot dike showing on a smaller scale the char- 

 acters noted in the preceding; and many other similar structures are to 

 be seen, but nowhere, so far as observed, do they exert any perceptible 

 alteration effects upon the slates and quartzites into which they have 

 been injected. 



JOINTS AND FRACTURES 



Jointing is everywhere one of the most conspicuous characters. The 

 sharp ridges which, through the slate belt, are a conspicuous physio- 



