COHESIVE ATTRACTION. 625 



solid limestone out of corals or ordinary marine shells, and hence 

 their formation at great depths impossible. 



6. The great extent and thickness of single reefs. 



7. The action of tidal currents and those arising from the piling 

 in of the waves during stormy weather, in keeping open chan- 

 nels and harbors, and determining the distribution of the coral 

 detritus. 



8. The close proximity, along shores bordered by barrier-reefs, 

 of deposits of coral material, and deposits of river or ordinary shore 

 detritus. 



9. An exceedingly slow subsidence in progress during the growth 

 of the corals the cause of the change of a fringing reef into a 

 barrier, and ultimately into an atoll. 



10. The necessity of this subsidence for giving great thickness 

 to such limestones. 



II. COHESIVE ATTRACTION— CRYSTALLIZATION. 



The power of cohesion acting in solidification and that in crys- 

 tallization appear to be identical. Snow, ice, bar-iron, trap, gra- 

 nite, and even solid spermaceti, are crystallized in their intimate 

 structure. Iron and granite show it in the angular grains which 

 make up the mass, and which may be observed on a surface of 

 fracture ; and ice, in the frosty covering of windows, and the prisms 

 which shoot across a surface on freezing, as well as in the vertical 

 columns into which it sometimes breaks when the ice of a pond 

 melts in spring. Quartz exhibits it in its prismatic and pyramidal 

 crystals (p. 55). The fact can thus be proved for all mineral 

 solids, except it be those of a glassj nature ; and even these are 

 probably no exception to the principle that solidification is crystal- 

 lization. 



Crystallization is exhibited (1) in the angular solids it produces, 

 called crystals, and (2) in a tendency to cleave or divide in one or 

 more directions, called cleavage. 



Crystals. — Some of the forms of crystals are illustrated on the 

 early pages of this work (pp. 55-65). Crystals are formed when 

 substances cool from fusion (as when melted sulphur cools) ; or 

 solidify from solution (as in the evaporation of a solution of alum) ; 

 or become condensed from the state of vapor (as in the formation 

 yf snow from vapor of water). But it is requisite usually for per- 

 fection that the process should go forward with extreme slowness. 

 Free from all disturbing causes, and with space for the crystals to 



41 



