34 PHYSICAL AND CHEMICAL PROPERTIES OP BOCKS 



intermingled with occasional fresli-water shells and plant debris. 

 As it flowed quietly from the mouth of the pipe and spread out 

 over the surface, the clayey particles began quickly to separate 

 from the siliceous sand in the form of concretionary balls, which 

 in the course of a very short time would grow to be several 

 inches m diameter. Such, owing to the rapidity of their for- 

 mation, contained a large amount of sand and shells, though 

 clayey matter predominated. 



In crystalline rocks conci'etionary structure is less commonly 

 developed. Cases such as shown on Plate 7 are unique, and 

 in the case of the orbicular diorite of great interest on account 

 of the beauty of the stone and its adaptability for small orna- 

 mentation. 



Concretionary structure of a secondary nature may be de- 

 veloped through the process of weathering. Thus, by the oxi- 

 dizing action of meteoric waters percolating through a porous, 

 sand or sandstone, included nodules of iron disulphide (pyrite) 

 may be converted into an oxide which gradually segregates in 

 zones about the original nodule. This oxide, by its cementing 

 action, binds the grains together in the form of a hard crust, 

 leaving the central portion, formerly filled by pyrite, either 

 empty or occupied by loose sand.^ A zonal banding closely simu- 

 lating concretionary structure is common in rocks more or less 

 weathered and decomposed, but which is due not to original dep- 

 osition or crystallization of mineral matter about a centre, but 

 rather to the weathering of jointed blocks, the various chemical 

 agencies acting from without inward. 



A botryoidal structure is not uncommon among rocks and 

 minerals of chemical origin. It is, as a rule, confined to such 

 as are amorphous or radiating crystalline aggregates of a single 

 mineral, as chalcedony or the hematite iron ores. (See Fig. 

 1, PI 8 ) 



A brecciated structure, produced by the presence of angular 

 fragments in a finer ground, is of common occurrence among 

 fragmental rocks, but is more rare among the crystallines. It 

 is sometimes produced in volcanic rocks by the imbedding in the 

 still pasty magma of angular fragments of previously consoli- 

 dated material, as shown in Fig 2, PL 4. Columnar structure, 

 though comparatively common as the structure of a geological 



^Bee On the Formation of Sandstone Concretions, Proceedings U. S. Na- 

 tional Mnseum, Vol. XVII, pp. 87, 88. 



