788 



DYNAMICAL GEOLOGY. 



1142. 



iPPlf 



■ 



N. 



(8.) Synclinals and anticlinals, as in Fig. 1140, making but two strata of the whole, 

 1, 3, 5, being one, and 2, 4, the other, the latter the underlying one of the two ; and 

 each is doubled on itself. 



(9.) Fig. 1141 the same as last reversed, the anticlinals becoming synclinals, and the 

 synclinals anticlinals. 



(10.) Synclinals and anticlinals, as in Fig. 1142, in which there are three strata, 1 and 



5 being the same, and each, as well as 3, folded 

 on itself, while 2 and 4 are the same stratum. 



(11.) The same reversed, making 1 an anti- 

 clinal and 3 a synclinal. 



( 12. ) There may, further, be double flexures, 



that is, two synclinals and an anticlinal, or two 



anticlinals and a synclinal, in either one or each 



of the strata; or (13), the whole series, from No. 



» 1 to No. 5, may be half, or less, of an overturn 



/ fold, either anticlinal or synclinal. 



The problem with regard to the relations of 

 such a series of strata is to be solved by comparing them as to fossils; or if they contain 

 none, b} r looking over the region for places where the folds, instead of being steep, are 

 shallow synclinals and anticlinals, giving to the eye the true order of superposition, and 

 multiplying such observations to settle the question that arises as to an overturn fold; 

 studying the resemblances in texture, color, thickness, etc., in the different strata 1,3, 

 5, or 2 and 4, and noting differences or resemblances in the two sides of the same stratum, 

 or in the succession of the different parts composing them. To remove all doubts where 

 there are no fossils is often difficult. The difficulty is increased by the fact that a 

 stratum may vary its thickness, or thin out in one direction or the other, and also vary 

 greatly in its mineral constitution. The fact of conformability or not is always to be 

 studied ; but there may be apparent unconformability from flexures in the line of 

 strata, and from faults, which is no mark of difference of age. 



The greater flexures ill such a region are of all sizes, from a fractiou 

 of a mile to twenty miles or more ; and there are minor crumplings of 

 a few inches or feet in breadth. The flexures have a general paral- 

 lelism to the axis of the mountain range. Instead of one fold for the 

 range, or a like length to parallel folds, there is generally a succession 

 along the mountain region, like the series in the case of fissures illus- 

 trated in the figures on page 19, the several folds in the series over- 

 lapping one another. The flexures have rarely, if ever, the axial line 

 horizontal (a result that would require the utmost evenness in the beds 

 aud in the action of the flexing force) ; and besides being inclined, the 

 axes of adjoining flexures often incline in opposite directions, a nec- 

 essary mechanical effect in the process of warping. 



Further: in the folds of a mountain region the axial plane (see fig- 

 ures on p. 93, and the section, p. 786) is seldom vertical, their oppo- 

 site slopes in a transverse section, being unlike. The folds are thus 

 unsymmetrical ; but those of one region have the steeper side gen- 

 erally facing in the same direction. Besides, the mountain range has 

 a corresponding want of symmetry ; a side of steeper folds, and an 

 opposite in which they are gentle or gradually fade away. 



These points are illustrated, not only in the Appalachians and Green 

 Mountains, but also in the Jura Mountains, the Alps, Apennines, and 



