208 [Assembly 



live of little or no effect, while at other points where the decom- 

 position is far advanced, the two minerals are found in more 

 nearly equal proportions. 



Feldspar, it is well known, is a mineral very prone to decom- 

 position, as in the ease of those varieties affording the kaolin and 

 other clays used in the nianufacture of porcelain. In the present 

 instance the feldspar decomposes first and, by its decay, sets free 

 the plumbago, rendering it more or less porous, so that the frost 

 and rain more easily crumble it to a gravel, more or less fine in 

 proportion to the original quantity of the feldspathic cement. 

 This gravel, by a continuance of the same process, becomes the 

 dark colored and highly productive soil, the peculiar properties 

 of which have been already noted. 



These focts possess a much higher interest, when we consider 

 in connection with their practical value, the chemical principles 

 which have combined to produce the effects noticed : 



Plumbago or Graphite (as it is often termed) consists mainly 

 of carbon, with a variable quanity of Iron, with which it is 

 supposed to be mechanically combined ; when crystalized it is 

 nearly pure carbon, iu a very compact state, it is quite infusible 

 whence its value as a material fwr the manufacture of crucibles 

 and other utensils destined to support a high temperature, al- 

 though it may be burned provided the air have free access. Its 

 composition as found hy analysis is as follows : 



Carbon, 9^.4 



Oxide of iron, 1.4 



Silica, 2.6 



With variable proportions of water, it may however, be practi- 

 cally considered as carbon, the other ingredienis being scarcely 

 in sufficient quantity to produce any sensible effect in the soil. 

 This carbon as first liberated by the gradual decomposition of 

 the beds of plumbago is, owing to its compact state, quite inso- 

 luble, and until the processes of decomposition shall have re- 

 duced it to a very fine state of division, it may be said to be 

 quite inert in the soil, or to have at least only a mechanical action, 

 as a loosener, making the soil lighter and more pulverulent, and 

 rendering it more absorbtive of heat and light. As soon however as 



