Van Hise and Hoshins — Pre- Cambrian Geology. 213 



conformable. It is shown that unconformities are usually at 

 least regional in extent, that some of them are continental, and 

 that perhaps a few may even be intercontinental. As to the 

 time represented by an unconformity it is concluded that a 

 slight unconformity may mark no more than a minor part of a 

 period ; that a great unconformity probably always marks at 

 least a major part of a period ; and, finally, that a slight or 

 great unconformity may mark eras of time. 



In structural work in nonfossiliferous rocks it is concluded 

 that unconformities give the best datum horizons from which 

 to build up the stratigraphy in a geological province. Within 

 a geological province, next in value to unconformities is like 

 sequence of lithological formations in different districts. Of 

 the least value in stratigraphy is the lithological character of a 

 formation. Formations may vary, rapidly in character, or in 

 mediterranean seas may have a like original character for long 

 distances. It cannot be assumed, in passing from province to 

 province, that the unconformities are equivalent, and hence we 

 are yet without any criteria for correlating pre- Cambrian series 

 in different geological provinces. In the future the only ways 

 known in which the questions of correlation between geological 

 provinces in pre-Cambrian rocks can be certainly solved are to 

 iind fossils in the series concerned or to work out the physical 

 history of the continents, a'nd the more hopeful direction of 

 attack is along the line of the physical history of the continent. 



Part II. — Historical Geology. 



The character, origin, and delimitation of the Archean are 

 considered. The conclusion is reached that the Archean is 

 igneous, and represents a part of the original crust of the earth 

 or its downward crystallization. The Algonkian is defined as 

 including all recognizable pre-Cambrian elastics and their 

 equivalent crystallines. The character and delimitation of the 

 Algonkian are considered. The rock succession, correlation, 

 and principles illustrated of each of the pre-Cambrian regions 

 of North America are summarized. The areas considered are 

 the Original Laurentian district, the Adirondack district, the 

 Hastings district, the Original Huronian district, the Lake 

 Superior region, the Great JS^orthern region, Eastern Canada, 

 isolated areas in the Mississippi valley, the Cordilleras, and the 

 eastern United States. 



In an appendix on Flow and Fracture of Rocks as Related 

 to Structure, Prof. Hoskins considers the conditions of flow 

 and fracture of rocks, strain and stress, and the applications of 

 the latter to rock structures. As the entire discussion is a 

 summary of a very difficult subject, any attempt to abstract it 

 would probably lead to misconceptions, and, therefore, the 

 attempt is not made. 



