THE IRON-BEARING MEMBER. 207 



veiulets have very varying extents, and have often severed concretions into 

 several fra<*'nients. In the concretions the iron oxide is arranged in concentric 

 oval or spherical bands, and this is eqnally true wliether tlie iron oxide is 

 linionite, hematite, or magnetite. This iron oxide is arranged without the 

 slightest reference to the individuals of quartz cutting through them in the 

 most indiscriminate manner. The concretionary structure affects the silica 

 both in concentric arrangement and coarseness of individuals. It is quite 

 common for the individuals of quartz to be larger in the concretions than in 

 the cherty matrix. (PI. xxii. Figs. 1 and 2.) Rarely these concretions have 

 nuclei from extraneous sources, as, for instance, small particles of fragmental 

 quartz, of a comparatively large individual or a cluster of individuals 

 of quartz, or else of iron oxide, but ordinarily they have none. Some- 

 times the concretions are so closely clustered that in their growtli they 

 interfere, and in such cases two or more concretions are used as a inicleus 

 about which the bands of iron oxide arrange themselves concentrically, 

 thus forming compound concretions. (PI. xxvii. Fig. 3.) 



A less prevalent characteristic of these rocks than the concretions 

 consists in the extraoi-dinary brecciated appearance which they present. 

 (PI. XXII, Figs. 2 and 3 ; PI. xxiii. Figs. 1 and 2.) A similar appearance has 

 already been noted as characterizing the chert of the limestone. The out- 

 lines of the fragmental areas are very commonly more or less sharply 

 angular, while frequently convexities and projections in the outline of a 

 detached fragmental area coiTespond to concavities and recessions of out- 

 line of another fragment in such a manner as to demonstrate a former 

 continuity of the two. The concretionary areas, in which, it should be 

 remembered, silica is still the main inOTedient, occur along- with the frasr- 

 ment-like areas, while between the two there is found such a complete series 

 of gradations that it is impossible to resist the conclusion that in many 

 cases both are of the same origin, only the brecciated phases have been 

 shattered by dynamic movements. 



The great variety of forms presented b)- the different sections fortu- 

 nately relate the history of these concretionary and brecciated areas. (PI. 

 xxvii.) In the first place, entirely unaltered areas of iron carbonate are 

 found associated with these concretions. . Other iron carbonate areas lying 



