LESSONS IN GEOLOGY. 349 



suited in adding many soluble chlorides and sulphates to the 

 sea-water and in the formation of sediments as silica, silicates 

 of aluminium, potassium, calcium and sodium. These materials 

 with the original crust make up a system of granites, gneisses 

 and crystalline schists, which represent the oldest rocks of the 

 earth. These rocks are doubtless of a mixed origin partly igne- 

 ous, partly sedimentary, but they have been so completely 

 changed by metamorphic agencies that all trace of lava, slag, 

 and sediment has been obliterated. 



These rocks, sometimes called the " Basement Complex," are 

 found in Arizona, Nevada, and Texas, in the regions of Lake 

 Superior and Hudson's Bay, including the greater part of theLau- 

 rentian rocks of Canada. These rocks were subjected to the 

 folding, crumpling and faulting incident to mountain formation 

 and to atmospheric erosion, and have been covered extensively 

 by later rocks so that we can know little of their original form 

 and extent. The time from the beginning of the crust to the time 

 when rocks that are distinctly sedimentary began to be formed 

 is called the Archaean or beginning era, and the rocks belong to 

 the Basement Complex, or Laurentian Age. 



The AlgonkianEra. Under the action of erosive agencies large 

 portions of the Archaean rocks were broken down and distributed 

 over adjacent sea bottoms as sediments, which afterward were 

 changed into conglomerates, quartzites, quartz, mica and iron 

 schists and limestones. These rocks vary considerably, pass- 

 ing from quartzite to quartz-schist; from conglomerate to con- 

 glomerate schist ; from pure limestone almost like marble, to lime- 

 stone containing great quantities of chert; and with the iron 

 schists there are cherts and jaspers in abundance. These rocks are 

 called the Lower Huronian. At the end of Lower Huronian time 

 this region was raised above the sea, crumpled and folded, and 

 subjected to erosive agencies, which in some instances carried 

 away the whole series. After this period of erosion the surface 

 was depressed and sediments were deposited on the eroded sur- 

 faces that afterward were changed into the conglomerates, quartz- 

 ites, shales, schists, iron schists and slates, chert, jasper, etc., 



