EARTH FEATURES AND THEIR MEANING 



solution, or solution and redeposition of mineral matter may go 

 on together within the mass of the same rock. One new mineral 

 may have been produced from the dissolved materials of a num- 

 ber of earlier species, or several new minerals may 

 be the result of the alteration of a preexisting min- 

 eral with a more complex chemical structure. Where 

 the new mineral has been formed " in place," it has 

 sometimes been able to utilize the materials of all 

 the minerals which before existed there, or it may 

 FIG. 13. Crys- have been obliged to inclose within itself those earlier 

 > garnet cons tituents which it could not assimilate in its own. 



developed in 



a schist with structure (Fig. 13). 



grains of At other times a crystal which is imbedded in 

 cUd'ed be" roc ^ ^ as ^ een a ^acked upon its surface by the per- 

 cause not as- colating solutions, and the dissolved 

 eimiiated. materials have been deposited in place 

 s a crown of new minerals which steadily widens its 

 zone until the center is reached and the original 

 crystal has been entirely transformed (Fig. 14). It 

 is sometimes possible to say 

 that the action by which 

 these changes have been 

 brought about has involved 

 a nice adjustment of supply 

 of the chemical constituents 

 necessary to the formation 

 of the new mineral or min- 

 erals. In rocks which are 

 aggregates of several min- 

 Fio. is. A new mineral eral species, a newly formed 



FIG. 14. A 

 crystal of aug- 

 ite within the 

 mass of a rock 

 altered in part 

 to form a rim 

 of the min- 

 erals horn- 

 blende and 

 magnetite. 

 Note the orig- 

 inal outline of 

 the augite 



rim" between the mineral tne Common margin of Cer- crystaU 



having irregular fractures tain of these species, thus showing that 



white^mine^al ^Hme-soda ^Y su Pply those chemical elements which 



feldspar). were necessary to the formation of the 



new substance (Fig. 15). Thus it is seen 



that below the earth's surface chemical reactions are constantly - 

 going on, and the earlier rocks are thus locally being transformed 

 into others of a different mineral constitution. 



