39Q 



HISTORICAL GEOLOGY 



and deformation must be taken advantage of in making provisional 

 correlations. 



Characteristics of Archaeozoic Rocks. — No rocks are more complex 

 than those of this system. In fact, their very complexity is a char- 

 acter which aids in their determination. In the Lake Superior re- 

 gion (Fig. 366) the system is, in general, composed of a great series 

 (Keewatin) made up predominantly of dark-colored (basic) schists 

 and great masses of granitoid gneisses and light-colored (acid) schists 

 (Laurentian) which have apparently for the most part been intruded 

 into the Keewatin schists. The Keewatin schists are, therefore, as 

 far as present investigation shows, the oldest rocks of the earth's 

 crust (unless some of the gneisses prove to be of even greater age). 



They are composed 

 largely of lava flows 

 and tuffs, with oc- 

 casional conglomer- 

 ates, shales, and 

 beds of iron ore, 

 which have been 

 folded, contorted, 

 and so metamor- 

 phosed that their 

 former condition is 

 with difficulty rec- 

 ognized. They have, moreover, been broken by faults and by 

 massive intrusions. Dikes through which was forced the lava that 

 flowed over surfaces that have since been worn away now cut both 

 the schists and the Laurentian granites and gneisses. Great 

 batholiths (p. 328) of granite (Laurentian) occur so frequently as 

 to make them almost characteristic of the Archaeozoic systems, 

 and in certain regions they constitute the larger part of the surface 

 rock. These batholiths have, in turn, been broken, faulted, and in- 

 truded by lavas of later age, and these by even younger intrusions. 

 Formerly, before they were recognized as intrusive masses, the 

 granites and gneisses of the Archaeozoic systems were considered to 

 be portions of the original crust of the earth. That surfaces must 

 have existed upon which the lava flows and ash deposits spread and 

 from which the material was derived to form the sedimentary beds 

 is obvious. Nevertheless, no such surface has yet been recognized 

 with certainty, either because it is still buried beneath the overlying 



PfiOTEffOZOfG 



/IRCH£OZO/C 



Fig. 366. — Block diagram showing the occurrence and 

 complicated structure of Pre-Cambrian rocks in the Lake 

 Superior region. 



