526 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i, 



importance for the local observer but are without Aveight in a 

 discussion of the whole ; while there are others, apparently unim- 

 portant, of which the importance cannot be exaggerated. The diffi- 

 culty is to distinguish essentials from non-essentials, for one's 

 convictions long entertained exert much influence. In any case, 

 this personal equation must affect interpretation of the record, so 

 that the student must be content to offer only a contribution, leaving 

 to another the completion of the work. It is necessary first of all 

 to have a knowledge of present conditions. 



Fayol,^^^ in the third part of his work, has discussed the con- 

 stitution, origin and formation of deltas, as bearing upon the deposit 

 of sedimentary rocks and has given the results of experiments in the 

 extensive settling vats at Commentry. After exposing the errors 

 into which geologists have been led by imperfect observations in too 

 limited areas, he proceeds to show that the lack of horizontality is 

 not necessarily evidence of dislocation, first referring to Weg- 

 mann's^^'' experiments on deposit of sediments upon an inclined 

 surface. Where beds have been laid down on such surfaces, they 

 pass by insensible slopes and curved lines from horizontality to 

 inclinations more or less strong and modeled on the surface below ; 

 the same beds, thick in the low parts, become thinner in rising and 

 imbricate with beds previously existing. In discussing delta de- 

 posits, one must distinguish carefully between the Alluvial or 

 emerged and the Neptunian or submerged portions. The material 

 may be either coarse or fine and the inclination may vary con- 

 siderably. 



Lake deltas, such as are seen in alpine areas, have abrupt 

 affluents and the material near shore is coarse while fine stuff is 

 beyond. The slope at the upper edge is from 25 to 35 degrees 



"" H. Fayol, fitudes sur le terrain houiller de Commentry. Litho- 

 logie et stratigraphie," Saint-Etienne, 1887, pp. 356-531. Though the writer 

 dissents energetically from many of Fayol's conclusions, he cannot withhold 

 the expression of admiration for the manner in which that author has recorded 

 every observation, making the work a treasury house of suggestion and in- 

 formation. It can serve as a model. If other observers had followed the 

 same method, one engaged in preparation of a monograph would not have 

 so frequent occasion to lament the folly which led him to begin the work. 



™BuU. Soc. Geol. de France, Vol. 7, 1850, p. 187. 



