m:i. . \Tinx or IXDIVIDCAI. CRYSTALfl 



135 



FIG. 269. Parallel Aggregate of Sulphur Crystals. 

 Girgenti, Sicily. 



It often happen- that t he faces of a large crystal may have a mat - 



like surface, caused by a layer of small individuals which have been 



deposited, generally 



in parallel position, 



upon the surface of 



the larger crystal. 



They either repre- 

 sent a secondary 



generation, or are 



the result of 



changed condition 



during crystalli/a- 



tion, causing a more 



rapid deposition, 



Fig. 271. 



Drusiness of faces 



is also produced 



either by a second 



generation of the 



same species, or by 



secondary minerals 



formed by the decomposition of the surface of the mineral upon 



which the small 

 crystals are 

 placed. 



Complete par- 

 allelism exists the 

 more often be- 

 tween individ- 

 uals of the same 

 species, or the 

 species of an iso- 

 morphous group. 

 The dividing line 

 between individ- 

 uals is not al- 

 ways distinct, for 

 as each individ- 

 ual is reduced in 

 size, each may 

 vanish as a line, 



FIG. 270. Parallel Aggregate of Microclitie from Pike's 

 Peak, Coloiado. 



