internal Structure of the Magnesian Limestone. 95 



is the first approach to a regular concretionary structure, and appears to have been effected by a 

 separation of parts after deposition ; for the harder portions of these honeycombed masses some, 

 times contain scarcely a trace of carbonate of magnesia, while the earthy portions contain it in 

 considerable quantity. 



2. On separating the beds (especially where the transverse fracture exposes the earthy spots 

 above described) in a direction parallel to the planes of deposit, we often find that the laminae 

 are not continuous, but made up of circular plates irregularly blended with, and running into 

 each other; the intervals between which are filled with a yellow magnesian earth. These plates 

 sometimes assume a discoidal form, and at first sight might be mistaken for lar^e Nummulites. 

 Here we have a distinct tendency to aggregation about diflerent centres ; but the operation being 

 confined to given laminae, could not develope itself in a vertical direction. The concretions were 

 therefore expanded laterally, and assumed a lenticular or flattened spheroidal form. That these 

 changes took place after the deposition of the rock, is rendered probable by the additional fact, 

 that the laminations of the beds may be often observed to pass (without any deviation) through 

 the various subordinate concretionary masses. 



3. At the planes of separation between two of these laminated beds, there is frequently some 

 earthy matter ; and the discoidal concretions are still more perfectly developed, and sometimes 

 approach the spherical form. These spheroidal masses impress both the upper and lower surfaces 

 of the beds with which they are in contact ; and we may find large concretions in such a position 

 that it is impossible to know to which bed we should refer them, although we may trace through 

 their substance the laminae and the lines which mark the stratification of the contiguous masses. 

 In such cases (of which I have seen examples near Black Rocks), it seems hardly to admit of 

 doubt, that the concretionary form was superinduced by some internal movement of the particles 

 after deposition. In the preceding instances the rock exhibits at the same time an earthy, a cry. 

 stalline, a laminated, and a globular structure. The following modifications are still more 

 remarkable. 



4. Several of the slaty or laminated beds (especially at the localities near Sunderland) are in. 

 terrupted by cells of considerable magnitude, which are of very irregular forms, but generally 

 elongated in the direction of stratification. Some of the smaller cells are nearly empty, and are 

 not so much coated with crystallized carbonate of lime as in many instances before mentioned. 

 But in general they are nearly filled with concretionary masses mixed with the yellow magnesian 

 powder. The concretions may be separated into three classes, which will be described in order : 

 — those which are aggregated on the floor and roof of the cells ; — those which are aggregated on 

 the sides ; — and those which are packed up in the middle of the powder and unattached to the 

 parent rock. On examining the floor and roof of one of these cells after (he powder has been 

 washed out, wc may find them studded with circular plates or spheroidal concretions resembling 

 the modifications last described ; but the process of aggregation being here uninterrupted, gives 

 rise to more complex results. We often find the concretions ascending from the floor, or hanging 

 from the roof in the form of elongated cones ; sometimes extended so far as to approach and 

 become interlaced with each other, and nearly to occupy the whole cell ; sometimes aggregated 

 from one of the surfaces and extending to the other in the form of irregular cylindrical or conical 

 pillars ; and, sometimes, exhibiting all these varieties of form mixed with, and interrupted by, 

 distinct spheroidal concretions, producing a structure too complicated for description. 



The surfaces of these concretions are sometimes smooth, in which case the internal structure 

 is imperfectly crystalline. Sometimes they are covered with crystalline points composed of the 

 acute solid angle of the inverse rhomb ; in which case the internal structure is crystalline, and 



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