138 



DYNAMICAL GEOLOGY. 



139. 



Quartzyte septaria. D. '84. 



140. 



rents ; moreover, the iron oxide spread either side, staining the rock, pro- 

 ducing the appearance of very wide rifts. Along one rift there is an open 



space from the loss of grains, and in it a crust 

 of newly formed quartz crystals. The process 

 often results in pushing the pieces out of place. 

 Where saline efflorescences — as alums, ni- 

 trates, alkaline carbonates, or chlorides — are 

 produced in the pores of a sandstone, the surface 

 grains are successively pried off. Much denuda- 

 tion is thus produced, especially in arid regions. 

 The process often makes a series of excavations 

 along the front of bluffs. The process goes on 

 most actively in covered places and during the 

 heat of the day. A shale often has its laminae 

 separated by layers of the salt or oxide, and fragments detached. 



Displacement by intrusion of crystalline material is a common process. 

 The following figure illustrates a case in which crystals of tourmaline in 

 mica schist are pushed apart at planes of fracture by intruding quartz (the 

 dotted portion) from a siliceous solution. After the first deposit of quartz 

 within the fracture, the additions were made between this deposit and the 

 adjoining part of the crystal, 

 and so the wedging apart 

 went on. A. H. Worthen has 

 described Crinoids, from the 

 Keokuk limestone, as split 

 open and enlarged in this 

 way, and one Barycrinus that 

 was thus made a foot in di- 

 ameter. The tubular stems 

 are increased four to six di- 

 ameters in the process. The 

 siliceous solution supplying 

 the quartz of the Keokuk 

 limestone was probably not 

 heated. 



The displacements may be great when large masses of a rock undergo 

 change to a kind requiring additional space. In the change of a bed of 

 anhydrite to gypsum the increase of bulk, due to the added water (page 128), 

 is nearly 60 per cent. Dividing the atomic weight of anhydrite, which is 

 136, by the specific gravity, 2-95, gives 46-1 for the bulk ; and that of gypsum, 

 172, by its specific gravity, 2-33, gives the bulk 73-8, making thus the gain 

 in bulk from 46-1 to 73-8. The change is hence attended by a breaking 

 and displacement of any overlying beds of rock. In the change of calcite 

 to true dolomyte, (i-Ca|-Mg) COg, there is a diminution in bulk of one 

 eighth per cent (or one tenth, if the composition is (|Ca |Mg) COg) ; which. 



Broken crystals of tourmaline displaced by intruded quartz, 

 Lenox, Mass. D. '85. 



