154 The Petrography and Genesis of Sediments 



Summary and Conclusion. — The prominence of fine-grained material 

 in the sample and the abundant mica and carbonaceous matter recall the 

 Magothy formation of this region, but it differs from the Magothy in the 

 field by occurring in massive beds, while the Magothy is thin-bedded or 

 laminated. Moreover, there are marked differences in the composition of 

 the material. Its diagram (G, p. 169) is peculiar in that while it shows 

 almost only fine material the nearly equal proportion of the different sizes 

 is striking. The abrupt rise of the " curve " on the right is a character, as 

 already noted, of stream sediments, but the stream sediments shown in 

 diagram M, p. 170, do not show so large an admixture of clay to sands. In 

 the study of this bed in the field a peculiar mottled effect of light and 

 dark-gray portions, which on close examination were found generally to 

 consist of cylindrical tubes of the light sand running at random in more 

 or less vertical directions through a matrix of the dark sand, was noted. 

 They did not resemble worn tubes which are generally solid cylinders, not, 

 like these, hollow cylinders filled with the dark material that surrounds 

 them. The interpretation which suggested itself at the time was that 

 the sand had been deposited in the midst of reeds which after their decay 

 had been replaced by clay but had bleached the sand around them. I think 

 this clue leads to a diagram which while not exactly like G, p. 169, yet 

 explains some of its anomalies. On p. 170 are two diagrams, G and H, of 

 materials from the same general lithologic belt in the Lagoon of Thau, but 

 H representing sediment deposited in a portion of the lagoon overgrown 

 with water plants. The effect of such a tangle of plants would naturally be 

 to produce less perfect sorting, and this is what we see in comparing dia- 

 grams G and H, p. 170, the extra fine portion having been increased at the 

 expense of the clay but without an increase, even wdth a slight decrease, 

 in the relative amounts of the portions coarser than extra fine. This 

 low proportion of these coarser sizes would naturally result from their 

 interception in the same way by the nearer-shore portions of the same 

 plant areas. As a result of these processes then, a diagram like I, p. 169, 

 though of the general lagoonal type, comes to resemble more specifically 

 diagram H, p. 170, the extra fine sand and a part of the clay having been 



