178 III. DIST. CHAR. Soil. 



of mineral matter being too thick to exhibit, ex- 

 ternally, the exact figure of the fossil which it 

 covers. Ironstone nodules are frequently subin- 

 crusting matrices. 

 ** Substance, 

 According to which the matrix is either 



a. 155. EARTHY (M. terrea). 



b. 156. INFLAMMABLE (J\I. inflammabilis). 



c. 157. METALLIC (M. metallica). 



Obs. The list of minerals already given,, unSer 

 the article seat, may be referred to for the sub- 

 stances which are found constituting matrices. 

 *** Parts. 



The parts of the matrix are its surface and the 

 impression of the organic body. 



a. 158, THE SURFACE (M. superficies] is the out- 

 side or exterior part of a distinct matrix This 

 may be rough, smooth,, cellular, chinky, drusy, 

 &c, &c. 



b. 159. THE IMPRESSION ( M. impressio) is the 

 interior part of the matrix, on which the form of the 

 inclosed body has been impressed f The matrix is 



t Linne, Wallerius, Cartheuser, and others, have considered 

 impressions, particularly of compressed bodies, as a distinct mode 

 of petrifaction ; but this is evidently incorrect Every organic 

 body, when first surrounded by mineral matter in a soft or plastic 

 state, must impart the form of its surface to the inclosing mass ; 

 and this form will remain, if it be not obliterated by the decay of 

 the organic body, before the investing matter has acquired that 



