III. DIST. CHAR. Soil. 177 



Amorphous (M. amorpha) without any parti- 

 cular form. Small masses of cliert, in limestone 

 beds., frequently afford examples of amorphous 

 matrices. 



Nodular (M. nodularls} having, more or less, a 

 rounded, or boulder-like form Examples,, nodules 

 of ironstone, flint, &c. These may be globular, 

 ovate,, oval, reniform, lenticular, &c. solid, lamel- 

 lated, tunicated, &c. 



Vein-like (M. leniformis) running in a thin, 

 plate-like form, through the solid substance of the 

 seat A vein-like matrix differs from a true vein, 

 which is sometimes the seat of the extraneous fossil, 

 in exhibiting only one compact substance, and in 

 being shut in on all sides by the solid rock; whereas 

 the true vein frequently holds a variety of sub- 

 stances, and, in a certain direction, is indefinitely 

 extended, (v. vein. p. 172.) Spar and iron pyrites, 

 sometimes form vein-like matrices. 



Incrusting (M. incrustaiu} covering the extra- 

 neous fossil in form of a thin coat or c.rust hence, 

 assuming the external shape of the inclosed body f 

 Example, tuff stone, &c. 



Subincrusting (M. subincrustans) differs from 

 the incrusting, in retaining only a slight appearance 

 of the general form of the inclosed body the coat 



t The common, calcareous incrustations of our rivers, &c. have 

 been ranked as petrifactions ; but, with more propriety, may be 

 considered as incipient matrices, 



A a 



