THE STRUCTURE OF THE OLAYiS. 703 



layers were cdutorted for a short distance in a most complex way, as 

 indicated in iig. 38, and on a larger scale in fig. 3IJ. 



It seems clear that the friction of the current was snflicient to slide the 

 layer of tenacious clay upon its substratum of fine sand for a short distance 

 and crumple it up, for each of these crumpled layers is covered by an 

 unusually thick and somewhat coarser film of sand. The layers grow 

 thinner toward the surface, and the upper 3 feet is an unctuous, nonlami- 

 nated clay. It is capped by the coarse beach gravel, which rises to the 

 surface. This is the onlv occurrence of sand beneath the clay I have seen 

 in the basin, though the fine sands of the Wapping cutting (see p. G95) are 

 so exactly e(piivalent to the clays that the red sands upon which the}' rest 

 uncomformably may come in the same categor}'. It is also the highest point 

 reached by the clays (251 feet), and here the till was for a short time swept 

 by a strong current from the main valley into the East Street basin before 

 the deposition of the clays began. 



THE STRUCTURE OF THE CLAYS. 



The upper horizontal lamina? in PI. XVI (p. 690), from Hatfield, illus- 

 trate tlie minute structure of the Cliani])lain clays. Over all the central 

 portion of the basin they are uniformly thin, even bedded, and horizontal, 

 show a regular alternation of fat and lean portions, and on drying separate 

 easily into layers, each of wdaich consists of a sandy part below and a fat 

 part above, which grade into each other. Tlie brickmakers call the "fat" 

 portion clay and the "lean" portion sand, distinguishing more closely than 

 the geologist. 



On the river bank at Hadley the lower and much the larger portion of 

 each layer is an extremely fine sandy clay, drab colored when wet, pale 

 buff when dry, composed of a fine, sharp, quartz sand, 0.15 to 0.24™'" in size, 

 and of kaolin in irregular elongated particles, afiecting reniform and sausage- 

 like shapes from flocculation. This passes rather abruptly, by the lessening 

 of the percentage of quartz grains, into an upper and finer portion, which is 

 generally one-fourth to one-fifth the thickness of the lower ^jortion, of dark 

 bluish-gray cohir when wet and olive green when dry. It contains a small 

 proportion of kaolin, the rest being very fine quartz grains. Its average 

 grain is 0.0008 to 0.0016"™ for the kaolin. 



In a specimen taken from the bank of Fort River, below ]\Iill Valley, 

 in Amherst, where the olive-green upper portion was 0.7™" thick, the coarser 



