6 



DEFINITIONS AND LAWS OF GEOLOGY. 



the intensity of any forces exceeded, in times past, those which are now in activity. 

 The changes which the earth has undergone within the scope of 

 geological investigations were produced by the same laws, acting 

 with the same degree of power, as those we may daily witness. 

 This is true of aqueous and igneous action and of all organic and 

 inorganic movements. 



§ 3. An anticlinal axis is that line from which strata dip to 

 either side. The ridge of a house-top, the slope of the roof rep- 

 resenting the dip of the strata, will convey an idea of an anti- 



io e wed U Cy n conforaa f b°ie clinal axis ; but an upheaval may be in the form of a dome, or 

 the arc of a circle, and, in such case, the strata incline in all 



Fig. 1. Anticlinal axis 

 at a. strata disturbed, 

 folded, and denuded, 

 and afterward uncon- 

 formable strata depos- 

 ited upon them, fol- 



A synclinal axis is the 



i''ni. X Bectlon of tin' Jan Mountains, Illustrat- 

 ing the folding of «i rata, anticlinal avis at A and 

 /(. Synclinal axis between .(and Sand betw i 



it iin'il C Strata unbroken at A and />', !>nt broken 



at 0, a, 6. c, and <i. strata conformable, though 



disturbed and thrown into waves. 



directions from a given point, which is the anticlinal axis 

 reverse of an anticlinal axis. Rocks are called 

 Gratified whether the planes of the beds are 

 parallel to each other, or rest unconformably. 

 Conformable strata have the planes of the beds 

 parallel to each other, and unconformable 

 strata have the planes of the strata of one 

 bed resting upon the edges of the strata of 

 another. This must necessarily mark an in- 

 terval of time between the two which is not 

 represented by a deposit. A fault is a dis- 

 location of strata so that the continuity of the mass is destroyed by one side of the 



fracture being elevated higher than 

 the other. A dyke is a wall of 

 rock between the two sides of a 

 fault or fracture, interrupting the 

 continuity of the beds on either 

 side. Sometimes a dyke shows an 

 overflow at the top. When strata 

 terminate abruptly, they terminate 

 in an escarpment. An outlier is a lone 

 rock in place, or a hill detached 

 by erosion from the surrounding 

 mass of similar beds, of which it 

 evidently once formed a part. 

 § 4. The erosion of the earth never ceases. Decomposed and disintegrated 

 substances are being constantly removed by rain and super- 

 ficial waters to a lower level than they previously occupied. 

 The erosion or denudation must be followed by the deposi- 

 tion of the materials. The deposit at one place can only 

 progress at the rate with which it is transported from 

 another. All strata consist of transported matter, and, as 

 Lyell said, the evidence of the work of denudation is Fl »- 4 - strata inclined but 



J ' conformable. Fault from/ to 



defective, because it is the tendency of every destroying 2 and dyke m the fault. 

 cause to obliterate in great part the signs of its own agency. Stratified rocks, 

 therefore, indicate only part of the erosion which the earth's surface has undergone, 



Fig. 3. Escarpment on the right, tlibrU representing the 

 slope from the escarpment. Outliers, lone rocks, or standing 

 columns in the center and to the left of the illustration. The 

 central figure is a form sometimes called a cheese rock. 



