THE PHYSICAL PROPERTIES OF CLAY. in 



be more intensely colored by the same quantity than one with 

 many clay particles. 



Iron oxide colors a clay yellow, brown, or red, depending 

 on the form of oxide present. The greenish color of many of 

 the Clay Marls is due to* the presence of the mineral glauconite. 1 

 The iron coloration is, however, often concealed by the black 



Fig. 32. 



Showing grains of sand in a Clay Marl I, enlarged 115 diameters. M, mica; Q, quartz; 



F, feldspar; I,. lignite. 



coloration due to carbonaceous matter. It is often more or 

 less difficult to make even an approximate estimate of the iron 

 content in a clay from its color. Thus, for example, two clays 

 from the Perth Amboy district which were of nearly the same 



1 See Iron, p. 57. 



