380 /. D. Dana on Cohesive Attraction. 



and produce a fibrous structure. Such a structure is common in 

 narrow seams in rocks, proceeding either from this cause, or 

 perhaps in part from the electric influence of the adjoining 

 walls of the seams. 



The concentric structure is another result depending on the 

 rate of solidification connected often with the rate of chemical 

 combination. In the first place the nucleus is always a cluster 

 of molecules, instead of a single one as for a simple crystal. 

 The structure sometimes commences around some foreign body 

 as a eentre, though the aggregation is often without any proper 

 nucleus, except that of the cluster of molecules that first solid- 

 ified. The second principle, on which the concentric structure 

 depends, is the tendency of a body to communicate its own con- 

 dition to other bodies within its influence. This law — the law of 

 equilibrium, and contact, or catalysis in chemistry — is one of the 

 universal laws of existence. According to it, either a collection 

 of molecules entering the solid state, or any foreign body already 

 solid will tend to bring adjacent bodies into the same or an in- 

 termediate condition. If susceptible to this influence, the parti- 

 cles adjoining become assimilated, and unite to the nucleus; 

 these again act upon others adjoining, and thus a spherical form 

 is produced, as a result of successive development. In glass 

 that has cooled with extreme slowness, there are often spherical 

 aggregations of crystals. Here, in some single point, the min- 

 eral of the aggregation first besjan to form ; and once begun, the 

 process was continued, according to this law of influence, around 

 the point as a centre, and the aggregations are therefore spherical. 

 In cooling basalt or granite, large spherical concretions are often 

 formed. The process of solidifying is in these cases continued 

 through a very long period of time ; and from the relation often 

 perceived between the thickness of a bed of basalt and the size 

 of the concretions, this size is evidently greater the slower the 

 cooling. In this prolonged cooling, after a while, here and there 

 a spot reaches the solidifying condition, and the process com- 

 mences. The particles adjoining, as explained, become solid 

 about the spot ; thence the process extends itself equally in every 

 direction ; and spheres are the result. The slower the cooling, the 

 longer the time occupied in passing through a single degree ot 

 temperature ; and consequently, when the cooling is most gradu- 

 al, the centres would subordinate to themH iro ° Q ,s,r!?e anl0U 

 of material, and produce larger concretions. 



uuuoiMu, cviiu ptuuuct; larger concretions. , 



In other cases, a solution is infiltrating through a clay or sand : 

 something (it may be a harder point or spot, or some organic ot>- 

 ject) determines the commencement of solidification at certain 

 points in the clay, and from this, the process continues by simp' 

 propagation, as just described. The stratification of the cla>, 

 or texture arising from gravity, favoring infiltration laterally mor 



