LAWS OF CONCRETION. 247 



condition, it has crystallized and produced hyaline quartz. But when it was in a gelatin- 

 ous condition, it has produced such silex as occurs in agates and chalcedony. 



" This theory of the formatidh of silicious concretions, may be reduced to the three 

 following points : 



" 1. The silex that forms agates and chalcedony was not in a liquid or aqueous solution, 

 but gelatinized. 



"2. In becoming solid it did not crystallize, as that which separated itself from aqueous 

 solution, and which produced hyaline quartz ; but it took circular and spherical forms 

 according to the position which it occupied. 



" 3. Organic matters appear to have had an influence on this secretion and this agglom- 

 eration of the silex." 



This hypothesis appears to us extremely probable, and we think it may be extended 

 further, so as to include a plastic as well as a gelatinous condition, and to embrace other 

 substances besides silex. This substance is, in fact, the only one that concretes, which 

 exists in a gelatinous condition sometimes. But silicates, carbonate of lime, hydrate of 

 iron, &c., are frequently in a plastic state, as well as in a state of solution. Our sugges- 

 tion is, that when in the former state, the regular bodies formed from it by molecular 

 attraction, will have rounded forms, but angular forms when in a state of solution. The 

 tendency of gelatinous silex is to the spherical form, as Brongniart has shown ; probably 

 it is the same with bodies in a plastic state, which is akin to the gelatinous. The reason 

 of this tendency has not been discovered. But if proved as a fact, it will account for 

 concretion. And we doubt not that taking this as a starting point, fixed laws may yet be 

 discovered, by which the almost endless forms of claystones were produced. Different 

 degrees of spissitude may have had something to do with the elimination of various forms. 

 Common clay could not have had a temperature much above that of the atmosphere. 

 But in the case of the nodules in granite, of concentric coats of mica and quartz, the heat 

 must have been quite high, and we doubt not that these concretions have passed through 

 a change of mineral constitution, analogous to the conversion of shale or sandstone into 

 mica schist. What the nodules were before the metamorphosis, we cannot conjecture. 



About twenty years ago the eminent microscopist, Prof. Ehrenberg, published the 

 results of his examination of concretions with the microscope. The title of his essay was, 

 On the Forms assumed by uncrystallized mineral substances, called Kidneys, Imatru Stones, 

 Claystones, &c. Some of these, the Imatru Stones, had been described by Prof. Parrot as an 

 extinct family of mollusks called Imatru. The Swedish claystones already referred to, are 

 known in Sweden by the name of Malrekor or Nockbroed, and were called by Linnaeus, 

 Tophus ludus and Merga porosa. 



These bodies, according to Ehrenberg, present no crystalline development, nor radiation, 

 nor organic arrangement in their structure. They show "a solid circle which is several 

 times repeated an evidently active development in their formation, and founded on uni- 

 form laws, and frequently, perhaps always, parting from many axes of formation." 

 "Generally we perceive two directions of development in the laws of their structure ; one 

 concentric and sometimes horizontal, and in one direction (sense) only, which constitutes 

 rays and discs, sometimes radiating on all sides, which produces spheres ; the other is 

 linear, and emanates from the center. For the most part these two axes exert a nearly 



