ALLUVIA. 629 



C. Of fine materials, consisting of sand and clay 

 more or less compacted. 



a. Clay. 



Many varieties, both in a mineralogical and 

 in an (Economical view, are comprised under 

 this head. It contains variable proportions of 

 alumina and of silica, and in some places, it also 

 contains magnesia and lime. Among the most 

 remarkable varieties which I have observed in this 

 country, are the white clay of north Uist and 

 Harris, and the porcelain clay found in some parts 

 of Cornwall in superficial beds ; the former 

 produced from gneiss, the latter from granite. 

 Although many of the clays described at page 608, 

 may resemble others belonging to the alluvial 

 deposits, their geological differences are impor- 

 tant. The colours are various, blueish black, 

 grey, yellow, or white. 



b. Clay with a large proportion of sand : 



loam of agriculturists. 



c. Compact sand, always with a mixture 



of fine clay sufficient to consolidate 

 it: found in the alluvia of rivers and 

 lakes, and on the sea shore. 



d. Clay containing inflammable or carbo- 



naceous matter arising from the de- 



