internal Structure of the Magnesian Limestone. 97 



appears as an upfilling matter in the intervening spaces. In some rare instances, theeartliy matter 

 becomes hard and coherent; and when broken with a hammer, exposes a surface wlilch passes 

 through the centres of the imbedded balls. 



In all the different varieties of concretions there are some which^ when 

 rubbed, exhale a hepatic odour. This circumstance is, however, accidental or 

 local. Carbonate of lime is their essential constituent*. During- the process 

 by w^hich the concretionary structure was effected, this mineral seems to have 

 separated itself almost entirely from the dolomitic earth, which is rejected, into 

 the ochreous powder. 



So far I have considered these various concretions in association with the 

 regular beds of the formation ; and the details seem to indicate that the several 

 forms have been superinduced upon masses which were originally stratified. 

 We have only to suppose that the same causes acted upon a more extended 

 scale, and we may imagine that the structure of whole systems of beds was 

 obliterated, and that they gradually passed into great piles of spherical con- 

 cretions like those which appear on the coast of Durham. 



A very complex modification of the concretionary form still remains to be 

 noticed. 



.5. In the quarries near Sunderland (for example, at Fulwell Hill), there are some strong thick 

 beds, in the greater part of which there is not a trace either of laminated, earthy, or globular 

 structure. Here and there we may, however, observe in them a kind of honeycombed appearance, 

 in which the small cells are not arranged in horizontal lines in the manner before alluded to, but 

 in concentric circles. These cells (which were originally filled with pulverulent matter) are the 

 intervals between small irregular discoidal concretions, having a concentric arrangement about a 

 hard spherical nucleus, which is commonly made up of an obscure congeries of spheres. In this 

 way are found, in the heart of the most solid strata, a number of spherical concretions (sometimes 

 nearly a foot in diameter, and having little appearance of a crystalline texture), each being 

 formed by the juxta-position of an indefinite number of small irregular discoidal concretions. 



As these small discoidal concretions not only touch but pass into each other, they give each of 

 the large spheres, especially on a fractured surface, the appearance of being made up of irregular, 

 wavy, concentric laminaj. The same bed which in one end of a quarry is homogeneous, in the 

 other is almost made of these singular concretions: and it deserves remark, that the concentric 

 rings of the dilFerent centres do not intersect each other. The several compound spheres com. 

 menced their aggregation at their respective centres ; expanded themselves till they came in con- 

 tact, and then mutually compressed, but did not penetrate each other f- Blocks composed of 

 these concretions, when struck with a heavy hammer, separate at a number of natural joints, 

 and fall into many irregular solids bounded by trapezoidal faces. When the separation is com. 

 plete, each solid contains within itself the elements of a distinct concretion ; and the trapezoidal 



* Carbonate of lime is, I believe, found in considerable excess in most of the beds coiUaining 

 the concretionary masses above described. The carbonate of magnesia, or more properly the 

 combined carbonate of lime and magnesia, is only a subordinate part. 



t See Plate V. fig.- 4. 



VOL. III. SECOND SERIES. O 



