

ORIGIN OF MOUNTAINS. 



803 



open fractures, when those from direct pressure alone would remain 

 closed. 



But in the movements of the rocks torsion has always been feeble 



Fig. 1158. 



compared with the direct action of the fracturing 

 force ; and the more feeble the more nearly would 

 one of the sets of fractures be predominant, and 

 have a course at right angles to the action of the 

 force. As a matter of fact, joints are commonly 

 parallel, or nearly so, to the axis of upheaval, or of 

 two or more upheavals, in a region ; and they have 

 been made in the course of the same slow move- 

 ments which made the mountains. 



The effects of pressure in breaking and distort- 

 ing fossils are also among Daubree's reproductions 

 of nature's work by her own methods. He has also 

 added to the illustrations of the origin of slaty 

 cleavage. It has been recognized by many, since 

 Sedgwick first taught right views respecting the 

 cleavage of slates, that the structure was due to 

 the flattening of compressible grains and air cav- 

 ities in the clayey material, and the putting into 

 parallel positions all flat particles, through the 

 action of lateral pressure. This view has been sus- 

 tained by Sorby's microscopic examinations, and 

 his experiments subjecting to pressure clay and 

 scales of oxyd of iron ; by Tyndall, who has ren- 

 dered beeswax, clay, and other substances, lami- 

 nated by simple pressure ; and later by Daubree, 

 who experimented with clay and scales of mica, 

 and obtained a perfect schistose structure. The 

 rolling and hammering of metals results in a lami- 

 nated texture, which fracturing or acids may re- 

 veal, when not otherwise visible ; and several fine examples are fig- 

 ured by Daubree. 



The fractures that have given exit to igneous ejections have often 

 had a width of 200 feet, and sometimes have exceeded this. Such 

 open fractures might be made in anticlinals, but seemingly not in syn- 

 clinals. A lateral pull rather than a lateral pressure is apparently re- 

 quired for their origin. 



The production of oblique planes of fracture, and an eastward dip, 

 in the great north and south Mesozoic dikes of the Connecticut valley 

 accords with the results obtained by Daubree in a synclinal bend, as 

 exemplified in Fig. 1155, p. 801; but how the many great fissures 

 came to be left open so widely for igneous eruptions needs explanation. 



Portion of a plate of ice 

 showing its fractures 

 (X|), From a pho- 

 tograph. 



