THE ORIGIN AND DESCENT OF ROCKS. 431 



Molecular rearrangements. — Besides these and similar changes that 

 involve additions and subtractions through the agency of percolating 

 water, the molecules of some of the rock constituents rearrange them- 

 selves, or the elements enter into new chemical relations; thus, pyroxene 

 may pass into hornblende by a change of the crystaUine arrangement 

 of the molecules. The change may sometimes be caught in progress, 

 the outer part of the crystal being hornblende (which when thus formed 

 is called uralite), while the heart of the crystal remains pyroxene. So 

 aragonite may pass into calcite. 



By changes of the foregoing kinds, many crystalHne rocks are much 

 altered. Some become chloritic from the development of the soft, green 

 hydrated mineral, chlorite, derived from the pyroxene, amphibole, bio- 

 tite, and perhaps other sihcates of the original rock. Others become 

 talcose from the development of talc, a very soft, unctuous, hydrous 

 magnesian silicate developed from the magnesian minerals of the original 

 rock. Soapstone or steatite is a rock composed essentially of such 

 secondary material. Serpentine is a rock made up of a similar secondary 

 mineral (serpentine) apparently derived from chrysohte (oH^^ne) and 

 other magnesian minerals. Epidote, a complex Hme-iron-alumina sili- 

 cate, often recognizable by its peculiar pistachio-green color, is derived 

 from other sihcates, and is rather common in many varieties of crystal- 

 hne rocks. MelapMjre is a name apphed rather loosely and variously to 

 certain altered basic rocks of the basalt family. Diabase is essentially 

 an altered dolerite. Nearly all the very ancient basaltic rocks show 

 notable degrees of alteration, even though they appear to have escaped 

 unusual dynamic conditions since their original formation, and hence 

 their alteration seems to have resulted chiefly from the operation of 

 unobtrusive agencies, chief among which is the circulation of water. 



The Salient Features of Rock Descent 



The foregoing processes by which primitive or igneous rocks are dis- 

 integrated and their constituents converted into fragmental material 

 may be said to constitute the descent of rocks in its fuller sense. Viewed 

 chemically, the great features of the process are (1) the breaking down 

 of the complex sihcates, and (2) the gathering of the resultant simpler 

 sihcates (mainly aluminum sihcates) into the silt and clay beds, (3) the 

 assembhng of a large part of the free acidic element (the quartz) into 

 the sand and gravel beds, and (4) the concentration of a large part 



