CLASSIFICATION. 245 



COMPARISON OF THE POWER OF CONCRETION WITH THAT OF 



CRYSTALLIZATION. 



"The analogies between this power and that of chemical affinity are remote and but of little interest. Presi- 

 dent Hitchcock has remarked on some of its analogies with crystallization. It is remarkable that they are 

 so many and striking, while the powers themselves are so unlike. Concretions are analogous to crystals in 

 having a normal or primary form, although but one form probably exists in all concretions. In the concen- 

 tric structure, they have a cleavage, which, like that of crystals, is parallel to the surface, of the normal 

 form. They are like crystals in having numerous secondary forms, although these are not probably the 

 result of inherent laws, but of external circumstances. They also, by coalescence, give rise to twin and multi- 

 ple forms, as do crystals, and many of them in consequence of an indefinite number of approximate centers 

 of attraction result in amorphous masses. More frequently than crystals, they include foreign substances 

 which probably modify the form. As the crystals of given localities often have some peculiarity in 

 a predominant form, the same is true, as was first observed by President Hitchcock, of clay stones, both of 

 the simple and of the multiple forms. As attachment to a support prevents the completion of a perfect 

 crystal in that direction, so we have seen a similar effect to result from the attachment of the nucleus to a 

 concretion. 



"The relations of the power of concretion to that of ordinary cohesion, are more intimate than to that of 

 crystallization ; it is rather a case of resemblance than of analogy, and perhaps they are only modifications 

 of each other. The tendency of ordinary cohesion, as is well known, is to form spherical bodies, when as in 

 drops of water over a dusty floor, or in globules of mercury, the cohesion between particles of a given body 

 is not interfered with by their cohesion with a foreign substance. May not the ordinary effects of cohesion 

 be the results of concretion with an indefinite number of centers, so that distinct concretions sustain the 

 same relation to the common amorphous bodies which distinct crystals do to bodies composed of crystalline 

 grains ? But we have introduced this subject rather to elicit, than to offer suggestions. It is obvious that 

 the description and theory of concretions constitute a subject, which, although perhaps less extensive than 

 that of crystalology, is as properly entitled to rank as a distinct science. 



CLASSIFICATION OF FORMS. 



" The classification of the forms of concretions, is not impracticable. The most important distinction is 

 that of simple and of complex forms. The examination and classification of the former division, as we have 

 seen, conduct us to some of the fundamental principles of the subject. The classification of the complex 

 forms is not susceptible of precision, but as in the case of the analogous aggregations of imperfect crystals, 

 it is serviceable for purposes of description. Their most striking characters appear in the number and the 

 mode of grouping of their constituents. Different modes of classification may be required for the different 

 classes of concretions those which arise from igneous fusion those which constitute or exist in limestone 

 strata the claystones and perhaps the ferruginous cylinders of clay, and other less numerous and miscel- 

 laneous examples. 



" The greatest variety of forms exists among claystones, and we have found the following arrangement 

 convenient, although groups cannot be established, which shall not gradually merge into each other. This 

 classification obviously is of little importance as exhibiting any principles, and may be compared to the 

 classification of imperfect aggregate crystals into columnar, granular, &c. with their subdivisions. It is not 

 even desirable to mingle specimens from different localities, since, as before remarked, the peculiarities are a 

 matter of much interest. But the various forms from a given locality may be conveniently arranged by 

 this system, with modifications in some cases. 



The term nuclear we have introduced to designate a mode of formation, in which a concretion, probably 

 after an interval in the process, becomes the nucleus of an external concretion, which is separated by a 

 depression from the nucleus. 



