234 GEOLOGY AND CHEMISTRY OF SOILS. 



the poorest of soils, especially when the pebbles and the sand 

 are mostly from quartz rock. They form, to a great extent 

 the silicrous soils of agricultural writers, and are generally 

 warm and dry, without the power of retaining the manures 

 which are placed upon them. 



2. The other variety of the diluvial soils, the argillaceous, 

 are exactly similar to those in the next class. They are 

 formed of clay and sand, and are the opposite of the gravelly 

 diluvial soils in their character, being heavy, moist, retentive 

 of manures, and of water. They are capable, however, of 

 being made the most fertile and valuable of soils ; as they 

 compose what are generally denominated clayey, and when 

 long cultivated, loamy soils. 



III. Tertiary soils. The tertiary rocks are alternate beds 

 of sand, clay and marl, generally arranged in horizontal lay- 

 ers, and often not hardened into solid rock. The clays or argil- 

 laceous earths seem to have originated from the argillaceous 

 minerals ; of which feldspar, mica and zeolite are the prin- 

 cipal. These, with the last described variety of the diluvial 

 soil, answer to the description of clayey soils, although soils 

 from the tertiary rocks include several varieties. The cagil- 

 laceoiis in which clay predominates, and the sandy which re- 

 semble the soils of the diluvium, are two important divisions. 



The tertiary beds, many of them, seem to have resulted 

 from the filling up of ponds and lakes which were sometimes 

 covered with fresh, and at others, with salt water; hence, 

 they are often composed mostly of carbonate of lime, and 

 are filled with fossil remains, especially of shell fish. But 

 the more common variety of this soil is the clayey, and this 

 varies from the stiff clays in which water and manures are re- 

 tained for a long time, and which are generally cold, wet and 

 unfruitful, to i\\e richest clay loams, in which there is just sulfi- 

 cient alumina to give them body, and to enable them to sup- 

 port the roots of the grains and grasses, for which crops they 

 seem best fitted. 



