October 1, 1915] 



SCIENCE 



451 



slight inclination to beds, either deforma- 

 tional or depositional. The terrace is an acline 

 interrupting a monocline which continues 

 with the same dip direction hoth above and 

 below the acline. 



The horizontal bed is rare because (a) beds 

 are generally laid down on a shore which is an 

 inclined surface, and (b) when the shore is 

 raised at the time of emergence some tilting 

 will usually result. Even if as a whole it is 

 flat, the upper and lower surfaces are likely 

 to have an inclination because of differences 

 in deposition, compacting or cementation. 



Monocline 



" Monocline " is a much-abused term. Some 

 authors use it for what Scott more discrimi- 

 natingly calls a monoclinal flexure — " a single 

 sharp bend connecting strata which lie at 

 difPerent levels and often horizontal except 

 along the line of flexure." A more desirable 

 use is that of Grabau, Chamberlain and Salis- 

 bury, and the Century Dictionary, who follow 

 the author of the term, H. D. Rogers, in apply- 

 ing it simply to beds " dipping in one direc- 

 tion." 



The monocline may be subdivided into three 

 primary types : 



1. The plane monocline — all surfaces having 

 a roughly similar degree of dip. 



2. The anti-monocline is a curved portion of 

 a monocline which is convex, when seen from a 

 point perpendicular to the general surface and 

 above it. This is a very common structure. It 

 is readily seen that it is analogous to an anti- 

 cline and would become one if the surface in 

 general were tilted to a more horizontal posi- 

 tion. Similarly, an anticline tilted sufficiently 

 becomes an anti-monocline. Orton, with this 

 aspect in mind, called it an " arrested anti- 

 cline." 



3. The syn-monocline is a curved portion of 

 a monocline which is concave when seen from 

 a point perpendicular to the general surface 

 and above it. It bears the same relation to a 

 syncline that the anti-monocline does to the 

 anticline. 



4. The monoclinal flexure. In addition to 

 these fundamental units there is the very 



common combination of an anti-monocline 

 passing directly into a syn-monocline below it. 

 For this the term monoclinal flexure has long 

 been used. To restrict the word monocline to 

 this structure as some geologists have done is 

 very objectionable. 



5. Half-fold. This same combination is 

 found in the half -fold, which is the whole sur- 

 face from the axis of an erect anticline to the 

 axis of an adjoining syncline; or if the anti- 

 cline springs from a plane, to that plane. 



Anticlines are divisible into the following 

 classes : 



1. The dome is a surface dipping outwardly 

 in all directions from a central point or line. 



2. The level axis anticline is one where the 

 surface is in general horizontal along the axis 

 of the anticline. A very elongate dome may 

 have a middle portion which is also a level 

 axis anticline. 



3. The plunging anticline is one having the 

 axis itself inclined. An elongate dome is made 

 up of two plunging anticlines, the plunges 

 being in general in opposite directions. As 

 stated above, a level axis anticline may inter- 

 vene. 



4. Nose. Two anticlines may cross each 

 other. This generally produces a more or less 

 marked dome at the intersection, which has 

 radiating plunging axes. The anticlines are 

 seldom of equal magnitude. If one of them 

 is very much less than the other, it is seen 

 merely as a wrinkle in the flank of the largei 

 one. Since these are very common and con- 

 fusion arises if they are called anticlines with- 

 out qualification, the descriptive name of nose 

 is proposed. A nose is a relatively minor 

 plunging anticline on the flank of a much 

 larger anticline or syncline or in a monocline. 

 It causes the isobaths to show a mere wave in 

 the down-dip direction. 



Synclines 

 Applying the foregoing distinctions to syn- 

 clines, we have the opposite terms — hasin, 

 level axis syncline, plunging syncline and 

 chute. 



