24 SHRINKAGE FISSURES OLDER ROCKS. 



wear away the softer beds ; yet the lode will swell in 

 the soft strata, and contract or " choke " in the hard, 

 suggesting that during the drying process the ar- 

 gillous beds have contracted laterally more than the 

 arenaceous (fig. 6, PJ. I.) It may be said that the 

 rocks may have moved up and down, and finally 

 stopped in their original position. This undoubtedly 

 seems to be the case in some instances, but in 

 most others it is highly improbable. In Salisbury 

 Crag, Edinburgh, the joints (faults) in the igneous 

 rocks are mere lines ; while in the associated 

 derivate rocks, they are marked by a greater or less 

 thickness of " fault-rock. " Similarly in a bank of 

 silt, we find that the argillous beds contract during 

 the drying process more than the arenaceous. 



Fissures and faults in strata are not solely due to 

 the unequal contraction of the different beds, as 

 many of them have been formed during the several 

 movements in the earth's crust. These latter move- 

 ments are generally supposed to be due to the interior 

 portion of the earth cooling and contracting more 

 rapidly than the outermost shell. The latter being, 

 in consequence, subjected to powerful horizontal 

 pressure, which continuing to increase with the pro- 

 gressive interior contraction, the exterior portion must 

 eventually yield in places. 1 Other faults may be 



i " Formation of the Features of the Earth's Surface." By Prof. J. le 

 Conte. American Journal of Science and Art. Third Series- Vol. iv., 

 pp. 345, et scq. 



