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The earliest or Laurentiaii rocks form wliat we may call the back- 

 bone of our continent. They are all crystalline, and consist for the 

 most part of gneisses, granites, limestones, schists, labradorites,quartzites 

 -and in some places altered slates. They contain, prominently among 

 minerals, apatite, graphite and mica with great beds of iron ore and 

 many others of great interest to the mineralogist. These rocks ai'e well 

 developed in the Chelsea hills and the country to the noi'th. Geogra- 

 phically and roughly speaking, they may be said to extend from Labra- 

 ibor along the north side of the St. Lawrence to Lake Superior and Lake 

 of the Woods, whence they trend away north-westerly and reach 

 almost the Arctic Ocean. Various theories as to the origin of these differ- 

 ent kinds of rocks have been put forth. For many years they were 

 regarded by most geologists as altered sediments entirely, which had 

 been recomposed from the debris resulting from the disintegration of 

 the first existing crust of the earth, through the agency of water or 

 the atmosphere, as well as by the action of the ocean, by which the 

 sands, etc., were redistributed and formed sedimentary layers, which 

 subsequently became metamorphosed into the gneisses, limestones, etc. 

 By others it is held that a great part of these rocks was formed by de- 

 position from a semi-fluid magma, and that they represent the true crust 

 of the earth without the agency of water, while other portions are true 

 altered sediments. Still others again hold that all Archean rocks, by 

 which term is meant generally, though the phrase is somewhat ambigu- 

 oas, all rocks devoid of organisms, thus including Huronian as well as 

 Laurentian, were formed of sedimentation, and that they are origin- 

 ally crystalline rocks, in part at least due to chemical agencies, their 

 crystalline character not being a superinduced but an original property. 

 In such a variety of opinions it seems hard to decide which should 

 have the preference, and while it is scarcely possible that the old wars 

 of the Neptunists and Vulcanists will ever revive in all the intensity of 

 early days, there is yet to be found in the statements of the advocates 

 of either theory plenty. of food for discussion. In solving such problems 

 the microscopist plays an important part by the examination of thinly- 

 sliced rock sections, from which their characters are in many cases read- 

 ily deciphered and their igneous or aqueous origin easily determined. 

 It is probable that both agencies have been largely exerted. Certainly 



