GEOLOGY AND GEOGRAPHY OF CLAY DEPOSITS 119 



out the section, and has been caused by slips or pressure from 

 above, as when the clay is covered by a thick delta deposit. 

 Prof. R. P. Whitfield has told the writer of instances where the 

 clay layers had been disturbed to a depth of several feet from 

 the surface by the weight of bowlders and large trees. 



In many instances there occurs a crumpled strip of clay 

 between layers which are entirely undisturbed ; this has been 

 actually observed by the writer to have been caused by slipping 

 of the clay. 



Clay concretions. — These are of common occurrence, especially 

 in the yellow clay. They are of varying form and size. Many 

 of them have a cylindrical hole in the center, which is lined with 

 carbonaceous material. The flat concretions are found parallel 

 to the layers of the clay, and in many instances at a depth from 

 the surface to which the roots penetrate. 



Those found at a greater depth did not have the central 

 cylindrical cavity. They are very abundant in the yellow cla} 

 at Haverstraw. Eoots penetrating the clay at this locality were 

 surrounded by lumps of clay in the form of concentric rings. 

 These might seem to indicate the method of formation d( scribed 

 by Prof. J. D. Dana (Manual of Geol., p. 628). Again in the 

 yellow clay near the surface at Coxsackie were found some forms 

 which were similar in appearance to what Dr J. I. Northrup has 

 described as rhizomorphs.* They may bo due to the roots which 

 penetrate the clay, absorbing water from it and rejecting the 

 contained lime, which deposits itself around the root forming the 

 hard rhizomorph. Their interior structure was crystalline. 



Another form of concretion is found in the delta sands at 

 Croton Point. It consists of botryoidal masses of sand, cemented 

 by oxide of iron. Some of them show the layers of deposition of 

 the sand. The concretions are usually small, but one mass was 

 noticed fully six feet long and four feet wide. 



Concerning the origin of these ooncretions various opinions are 

 expressed by different geologists. 



Organic remains are extremely rare in these clays. The writer 

 has discovered sponge spicules, probably referable to Hyalonema 

 or an allied genus, and which are figured. The following 



* Trans. N. Y. Acad. Sci., Oct. 13, :890. 



