MACROSGOPIO STEUGTURE 33 



or those which are due to segregating influences acting subse- 

 quently to the formation of the rocks of which they now form a 

 part. All are due to that peculiar and little understood ten- 

 dency which atoms or molecules of like nature so often manifest 

 in concreting or gathering in amorphous masses or concentric 

 layers about some foreign body which serves as a primary point 

 -of attachment. The extreme development of this tendency is 

 seen in crystallization. Under primary concretions may be 

 included the flint and chalcedonic nodules found in chalk and 

 the older limestones, the material of which was in part derived 

 from the siliceous remains of radiolaria and sponges. Such 

 sometimes occur in the form of lenticular nodules with or with- 

 out an appreciable concentric structure, lying in parallel layers 

 or beds, continuous for long distances. Clay iron stone, an 

 impure carbonate of iron, occurs characteristically in this form. 

 These latter often crack on drying and consequent shrinkage, 

 the cracks extending from within outward. In these cracks eal- 

 'Cite is subsequently deposited, whereby the nodule is divided up 

 into septa of a white or yellowish color. On being cut and 

 polished, these often form beautiful and unique objects. To 

 such the name septarian nodule is commonly given. (See Pig. 

 2, PL 8.) The carbonate of lime in inland lakes and seas may 

 become deposited in the form of thin pellicles about a minute, 

 perhaps microscopic nucleus, forming small, spherical bodies 

 which, when ultimately consolidated into beds, give rise to the 

 oolitic and pisolitic limestones. (See p. 125.) 



All primary concretions are not, however, chemical deposits; 

 but, rather, aggregates of mineral particles in a finely frag- 

 mental condition. 



Such are the clay concretions which are found in the beds 

 of streams and lakes, and which may so closely simulate animal 

 forms as to be very misleading. The manner in which concre- 

 tions of this nature are formed was shown in a very interesting 

 manner a few years ago during the progress of the work of filling 

 in the so-called Potomac flats, on the river front at Washington, 

 District of Columbia, For the double purpose of raising the 

 flats and deepening the channel, gigantic pumps were employed 

 which raised the sediment from the river bottom in the form 

 of a thin mud and forced it through iron pipes to the flats, 

 where it flowed out, spreading quietly over the surface. The 

 material of this mud was mainly fine siliceous sand and clay 

 4 



