506 GEOLOGY 



ervation of fossils. Furthermore, fossils are sometimes retained 

 in later formations that have been much disturbed and altered. It 

 is also true that some later formations which seem well suited to 

 receiving and retaining organic impressions are devoid of them. 

 Geologists are inclined to refer the scantiness of pre-Cambrian 

 fossils, and hence the apparent abruptness of the introduction of 

 the Cambrian fauna, to unfavorable conditions for fossilization 

 in pre-Cambrian time, combined with subsequent changes in the 

 rock. This makes the abruptness a matter of record, rather than 

 a matter of fact. 



Map Work. The following folios of the U. S. Geological Survey furnish 

 good maps for the study of the stratigraphy and the stratigraphic relations 

 of the Cambrian system in different parts of the United States. The texts 

 of the folios give some account of the physical history of the several regions: 

 Alabama, Gadsden; Arizona, Clifton; Colorado, Anthracite-Crested Butte; 

 Georgia, Rome; Maine, Penobscot Bay; Massachusetts, Holyoke; Michigan, 

 Menominee; Montana, Little Belt, Fort Benton; New Jersey, Franklin Fur- 

 nace; North Carolina, Mount Mitchell, Nantahala, and Pisgah; Oklahoma, 

 Tishomingo; Tennessee, Maynardville, and Morristown; Utah, Tintic; Vir- 

 ginia-West Virginia, Bristol, Harper's Ferry, Tazewell; Wyoming, Absaroka 

 (Crandall Sheet), Bald Mountain-Dayton, Cloud Peak-Fort McKinney, and 

 Sundance. 



