16-i SCIENTIFIC AGRICULTURE. 



m liters, such as woody fibre, humus, peat, ulmine, <kc. The 

 salts of lime arc also of great value as fertilizers. 



MARL. 



Marl is a compound of lime and clay, so intimately mixed 

 that their respective particles cannot be distinguished. The 

 exact process by which nature combines the two elements is 

 not known; for if clay and lime -be mixed together artificially, 

 they form a substance quite different from natural marl: &nd, 

 according to Timer, "it does not possess the faculty of losing 

 its aggregation when exposed to the influence of the atmos- 

 phere, and crumbling to dust like natural marl." 



The proportions of the two elements are various: sometimes 

 the lime predominates, sometimes the clay, and in some speci- 

 mens they are equal. When the clay predominates greatly, it 

 is called clay marl: when the lime predominates, it is called 

 Iim2 ra ii-1. M irl is found in considerable variety, both of com- 

 position and color: it assumes a blue, red, yellowish or whitish 

 hue, according to the oxides of iron, or other matters which it 

 may contain : it is found in greater or less quantities in almost 

 all countries, sometimes on or near the surface, and in other 

 cases at considerable depths in the earth. 



"It is confined [says Dr. Hitchcock,] to the alluvial and 

 tertiary strata, and differs from many varieties of limestone, 

 only in not being consolidated." It often contains salts of 

 potash and soda, fragments of shells, bones, and some vegeta- 

 ble matter: that which contains a large quantity of shells is 

 called shell marl: several other species of marl are described, 

 the most important and valuable of which is greenstone marl 



Nearly all the varieties, except stone marl, are easily pene- 

 trated and their particles separated by water: frost is also an 

 active agent in pulverizing it, it is therefore often laid on 

 land at the beginning of winter. Marl may be detected by 

 the acids, with which it effervesces and forms salts. It is 

 evident from the character and composition of marl, that it is. 



