478 E. D. Preston — Magnetic and Gravity 



The top of the Chico beds was not exposed. It lies beneath 

 the newer formations which occupy the middle portion of the 

 Sacramento valley. The apparent thickness of the series may 

 have been somewhat increased by faulting. 



Section on North Fork of Cottonwood Creek, Shasta Co., Cat. 



Shales containing Chico fossils 621 



Shales with this sandstones 180 



Shales 1908 



Conglomerates and sandstones with many Chico fos- 

 sils 914 



Thickness of Chico beds, 3623 



Shales including 10 feet of sandstone contains 



numerous Horsetown fossils 2870 



Shales with some sandstones contains numerous 



Horsetown fossils 1787 



Sandstone with coarse conglomerates at base of series 561 



Thickness of Horsetown beds, 5218 



Apparent total thickness of unaltered Cretaceous, 8841 



The top of the Chico as in the other section was not exposed. 

 The apparent thickness of the series may have been somewhat 

 increased by faulting, a condition suggested by the occurrence 

 of Chico fossils near the western limit of the Cretaceous rocks. 

 Aucella was not found in this section, and the basal member 

 of the series rests directly and unconformably upon the meta- 

 morphic rocks. 



U. S. Geol. Survey, Washington, D. C. 



Art. LXII. — Magnetic and Gravity Observations on the 

 West Coast of Africa and at some islands in the North 

 and /South Atlantic ; by E. D. Preston.* 



The value of certain magnetic observations depends on the 

 interval of time between the determinations : gravity observa- 

 tions, when their object is a more accurate knowledge of the 

 shape of the earth, depend for their value on the area over 

 which the stations are scattered. Coast, continental and island 

 stations, as well as those on mountains and plains, each have 

 their own particular evidence to furnish. It must be admitted 

 that this evidence is not always unanimous, but the lack of 

 unanimity may result from a real cause, inherent in the nature 

 of matter, or it may be only apparent and come from the dif- 



* Read before the American Association for the Advancement of Science, 

 Indianapolis Meeting, August, 1890. Published by permission of the Superin- 

 tendent of the U. S. Coast and Geodetic Survey. 



