368 C. W. HALL — KEEWATIX OF EASTERN AND CENTRAL MINNESOTA 



finely ciystalline kaolinic material as to be almost indistinguishable. 

 At Little Falls and Sauk Rapids olivine is seen in partiall}' decomposed 

 crj^stals. In these dikes olivine was once a very important constituent. 



THE GABBRO 



Gabbro is mentioned here because it occurs in Little Falls at the western 

 end of profile V, plate 30, from Little Falls to Taylors Falls, across the 

 southern and southwestern portions of the area under discussion, and 

 mapped on plate 29. A belt of gabbro bosses stretches from Little Falls 

 into the southern half of Stearns county. The}' are believed to be })ost- 

 Keewatin. They are throughout quite similar in lithologic characters; 

 hence the Little Falls outcrop may be taken as a type. This rock is a 

 biotitic gabbro. Labradorite, diallage, possibly hypersthene, olivine, 

 magnetite, and several alteration products derived particular!}' from the 

 decomposing hypersthene and olivine, mark the mineral habit of the 

 rock species. The texture is medium, and alteration is marked. 



VEINS AND VEINSTUFFS 



Around Thomson and Carlton, and similarly throughout the Saint 

 Louis River district, there occur a large number of veins. Most of them 

 are thin, and, save as they point to structural conditions, insignificant. 

 They are plainly veins of infiltration wherever any direct clue to origin can 

 be seen. In plate 32, figure 2, is seen a quartz vein from one to three inches 

 in width, which is involved in the crumbling of the rocks to an unusual 

 extent. In many of the microscopically narrow veins quartz is the 

 leading constituent, through which hornblende needles are projected into 

 the veinstuff from the edges of broken hornblende individuals. The 

 process was one of enlargement of the hornblende grains through the 

 attachment in crystallographic continuity of fibers extending the pris- 

 matic axes of the old and disrupted hornblende. 



There frequently are to be seen the attachments of quartz crystals to 

 the walls of the minute fissures, with their axial or c directions pointed 

 across the space, not in crystallographic parallelism but rather in one 

 general direction. As veins become broader, the arrangement of vein 

 contents becomes more complex, until when feet across they are made 

 up almost entirely of granuhir quartz, pegmatitic masses, intermingled 

 siderite, and in one or two instances segregated sulphides. An exam- 

 ple of the first named is seen at the bridge acr6ss the Saint Louis river 

 at Thomson. It stands nearly perpendicular and strikes quite nearly 

 with the slates through which it breaks. It conforms therefore in posi- 

 tion with the slaty cleavage of the region. Yet it is far from regular. 

 Its thickness varies, both horizontally and vertically. 



