246 CLASSIFICATION. 



The concentric structure differs in not having a depression between the parts, which appear to have 

 resulted from slight and regular intervals in the process. 



A. Simple forms. 



1. Without apparent concentric arrangement. 



2. Encyclica. 



a. with concentric structure. 



b. Nuclear. 



B. Primary compounds ; simple or encyclic forms, united in the direction of the horizontal axis, 

 that is, edgewise. 



3. Dimera, composed of two simple or encyclic forms. 



a. with concentric structure. 



b. Nuclear. 



4. Dccamera, with four to ten parts. 



a. Elongated, parts less confluent. 



b. " quite " 



c. not " " less " 



d. " " " quite " 



5. Polymera, with more than ten parts. 



" subdivided like the decamerous concretions. 



C. Binary compounds, united in the direction of the shorter axis, face to face, in two layers. 



This and the following may be subdivided as their parts are dimerous, trinierous, &c. 



D. Ternary, united in three layers. 



E. The less common examples of four or more layers may constitute the last division. 



The amorphous bodies resulting from confused concretion, with an indefinite number of centers, must be 

 rejected from such an arrangement." 



Concretions are the crystals of geology ; that is, they sustain the same relations to the 

 rocks, as crystals do to the simple minerals. But they seem to us not to have received 

 the attention from geologists which they deserve. No Abbe Hauy has yet appeared to 

 develop the fundamental principle of their formation. Few geological cabinets indeed, 

 contain any large collections of them. The 2,000 specimens in the Amherst Cabinets, 

 chiefly of claystones and ferruginous concretions, are much the largest number we have 

 seen in any museum, and we trust they will furnish some future investigator of this sub- 

 ject with the means of developing the laws which have controlled the formation of con- 

 cretions. They are mainly from Vermont and Massachusetts, and present an extraordin- 

 ary variety of forms. 



Why standard works in English contain so little upon concretions, save those of the 

 oolite and magnesian limestone, we know not. Naumann, in his German G-eognosic, gives 

 the best summary description of them which we have seen ; yet he says but little about 

 their mode of formation, and does not even mention the claystones. If they were as re- 

 markable on the continent of Europe as in this country, he would not have omitted them. 

 Thirty years ago, however, Prof. Alexander Brongniart gave an able essay Sur les OrMc- 

 ules siliceux, or silicious concretions, such as agates, in Tome mngt-troisieme of the Annales 

 des Sciences Naturelles. He endeavored to show under what circumstances silex would 

 assume a spherical o"r concretionary form, and when it would crystallize in an angular 

 form ; and he arrived at the following conclusions : 



"When silex has been completely dissolved, and by consequence is in a perfectly liquid 



