SEDENTARY MATERIALS: RESIDUARY DEPOSITS 293 



may sometimes be found. In some eases, too, the perforations 

 have been seen to undergo a gradual variation in size, and to 

 branch now and them, much as rootlets do. On the other hand, 

 it is probable that some of the perforations have had a different 

 origin, for in one case a small insect was found in one of the 

 little canal-ways. The clay is exceedingly tenacious, and hence 

 the perforations, once formed, would endure for long periods of 

 time. 



^* Another characteristic of certain portions of the clay is its 

 power of retaining moisture. It can rarely be found, even in 

 the driest season, unless exposed to the direct rays of the sun, 

 without visible moisture a few inches from the surface. The 

 regions where it is present are conspicuously less afEected by 

 drouth than adjacent localities where it is wanting. For this 

 reason it is a valuable sub-soil. 



'fragments of residuary rock are not uncommon in the deeper 

 portions of this earth. Of these, chert fragments are most 

 abundant, and occur scattered sparingly throughout the clay or 

 sometimes arranged in more or less distinct layers in it. Even 

 where they appear to be entirely wanting, the microscope often 

 reveals minute flakes scattered sparsely throughout the clay. 

 The larger pieces are more numerous near the basal portion of 

 the clay than higher up. 



'^It is natural to suppose that the residuary earths derived 

 from the decomposition of limestone would differ very notably 

 from those which take their origin from sandstones or from 

 shales or mixed crystalline rocks. Yet the difference is far 

 less than might be anticipated. There usually overlies the 

 sandstone strata a loamy earth not very far removed in char- 

 acter from that which mantles limestones. It is somewhat 

 more sandy, and consequently less cohesive, and presents the 

 opposite variations in vertical sections, becoming less cohesive 

 below, instead of more so. In the limestone region the toughest 

 clay lies next to the rock. In the sandstone regions the soil 

 graduates below into sand. The difference is most conspicuous 

 where the mantle has been washed and redeposited and mingled 

 with mechanically derived sand and secondary products, as 

 occurs in some of the valleys."^ 



The following analyses, in part from this same report, will 

 answer, in connection with those already given, to show the 



^ 6tli Ann. Rep. U. S. Geol. Survey, 1884-85, pp. 240-242. 



