628 DYNAMICAL GEOLOGY. 



are metamorphic, are massive. Pyroxene, for the latter reason, seldom 

 makes any but massive rocks, whether of igneous or metamorphic ori- 

 gin. Feldspar also usually makes massive rocks under both condi- 

 tions ; and often in spite of the presence of much mica. Quartz forms 

 massive rocks except when determined otherwise through its sedimen- 

 tary origin ; and the thin bedding of quartzose sedimentary beds is 

 often wholly obliterated during metamorphism. Massive structure is, 

 therefore, not necessarily evidence of igneous origin. Slaty rocks, like 

 roofing slate, often owe their fissile structure to pressure. (Seep. 794 

 for remarks on this subject, and on the schistose structure of mica 

 schist, gneiss, and related rocks, called often foliation.) 



(3.) Concretionary Structure. — Examples of concretionary forms 

 are given on pages 85-88. There is a general tendency in matter, 

 when solidifying, to concrete around centres. These centres may be 

 determined (1) by foreign substances which act as nuclei, or (2) by 

 the circumstances of solidification, which, according to a general law> 

 favor 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 

 begins at points through the mass ; and these points are the centres of 

 the concretions into which the mass solidifies. 



The concretions in the same mass are usually of nearly equal size : 

 hence, (3) the points at which solidification in any special case begins 

 are usually nearly equidistant. 



Iron-stone and calcareous concretions in beds of rock are examples 

 in which the concreting is due to a mineral solution in the stratum of 

 clay or sand. A solution containing silica may make siliceous concre- 

 tions : so also a calcareous or a ferruginous solution may be the con- 

 creting agent. The silica may come from infusorial remains in the 

 bed, and the calcareous materials of the solution from calcareous 

 grains, and each be taken into solution by permeating waters. 



A mineral solution (or any liquid) naturally spreads equally in all 

 directions through a sandy or earthy stratum, and makes, therefore, 

 spherical concretions ; but, in a clayey rock, it spreads laterally most 

 rapidly, and so leads to flattened concretions, the diameters varying 

 with the rate of spreading in the two directions. 



In a concretionary mass, the drying 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, as in Figs. 72, 73 ; or, it may undergo no solidifica- 

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

 process, form a ball within a shell, with loose earth between. The 

 conditions giving rise to hollow balls among spheroidal concretions 



