Prof. Sedgwick on the Structure of large Mineral Masses. 465 



discover a series of planes belonging to some known definite form of carbonate 

 of lime. In the well-known Fontainebleau rhombs^ the crystalline forces of 

 the carbonate of lime have had such power over the particles of siliceous 

 sand, as to re-arrange them in regular forms. In the cases I am considering- 

 such a change is out of question, because the stratified calcareous grits were 

 probably held together by great pressure, and were partially in a state of 

 solid aggregation at the very time the crystalline plates of carbonate of lime 

 struck in various directions through the intervals of their mass. All that it 

 is important to observe, is the fact, that these plates pass obliquely through 

 the laminae of stratification, and, consequently, that the structure is one of 

 the numberless instances of chemical changes produced, in stratified rocks, 

 after their deposition. 



When we examine a weathered surface of calciferous grit, we generally 

 find it exhibiting inequalities, which prove that the crystalline plates of car- 

 bonate of lime are not uniformly arranged through the mass. In short, the 

 portions in which these plates are most completely developed are in such 

 cases of the nature of irregular concretions. More rarely the chatoyant calc- 

 grit is seen in the form of flattened spheroidal concretions, ranged with more 

 or less regularity on lines parallel to the planes of stratification ; and in such 

 cases, the concretions themselves are not unfrequently traversed by these 

 planes. The best examples of this structure may be seen in the neighbourhood 

 of Peterborough among the calcareous flagstones abounding in the upper part 

 of the great oolitic system of Northamptonshire. Thousands of globular con- 

 cretions of calc grit (generally exhibiting the original laminae of stratification) 

 lie scattered upon the upland plains on the confines of Huntingdonshire and 

 Cambridgeshire, Whether they have drifted from the calcareous grits of the 

 middle oolite (coral rag), or from the back of the great oolite of Bedfordshire, 

 is not easy to determine. The well-known slates of Stonesfield also exhibit 

 fine examples of this structure. 



(3.) Globular Magnesian Limestone. — The phaenomena accompanying this 

 structure have been described by myself at great length in a paper published 

 in the Transactions of this Society*. It is by far the most beautiful example 

 of a spheroidal structure, superinduced on stratified matter after its aqueous 

 deposition, which is to be met with in any of our secondary rocks. This con- 

 clusion is controverted hy i\\e ?L\xi\ioY oi i\\e Principles of Geologi/^ on the 

 strength of a supposed analogy between the formation of the magnesian lime- 

 stone and certain travertines : but as the supposed analogy is wanting in a fact 



* Second Scries, vol. iii. p. 94—98. f ^ol. i. p. 303. 



