56 JOURNAIv OF THE 



The concretions are also distinct from the rock mass. 

 Figure 13, taken from a microscopic section, roughly 

 illustrates these points. 



Having' the general characters of pakeotrochis before 

 us, let us now compare it with the forms that it has 

 been thought to resemble and examine more carefully 

 the more minute details. 



(1) Is it a concretion? "There is a general tendency 

 in matter, when solidifying to concrete around centres. 

 These centres may be determined (1) by foreign sub- 

 stances which act as nuclei, or (2) by the circumstances 

 of solidification, which according to a general law, fa- 

 vor a commencement of the process at certain points 

 in the mass, assumed at the time. As the solidifying- 

 condition is just being reached, instead of the whole 

 simultaneously concreting, the process generally be- 

 gins at points through the mass; and these points are 

 the centres of concretions into which the mass solidi- 

 fies. 



" The concretions in the same mass are usually nearly 

 of equal size; hence the points at which solidification 

 in an}^ special case begins are usually nearly equi- 

 distant. 



"In a concretionary mass, the drN^ng of the exterior, 

 by absorption around, may lead to its concreting first. 

 It then forms a shell with a wet unsolidified interior. 

 The interior may then dry, contract, and become 

 cracked; or, it nia}^ undergo no solidification, and remain 

 as loose earth; or, it may solidify by the concreting 

 process, forming a ])all within a shell, with loose earth 

 between. "^ 



If the forms we are considering were even spherical, 



1. Dana's Manual of (icolo^^-, paiji^e C)2P. 



