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



These examples prove at least two classes of chemical changes in stratified 

 rocks^ — the first produced by the penetration of fluids containing some new 

 cementing principle — the second by the long-continued mutual action of con- 

 tiguous solid parts upon each other. But, besides these chemical changes, 

 most secondary deposits have been modified by two mechanical causes; viz. a 

 contraction of dimensions in passing to a solid form, and a change in the 

 relative position of the component parts, produced during periods of eleva- 

 tion. I shall first endeavour to illustrate some of the chemical changes. 



§ 2. Globular and Concretionary Structure. 



It appears to be an acknowledged principle, that when different substances, 

 in a state of extreme comminution, are mechanically mixed together, they have 

 a tendency to separate and re-arrange themselves in masses more nearly homo- 

 geneous. The separation of the pounded flint from the aluminous earth, in 

 the materials prepared for the potteries, has been several times quoted as an 

 instance of this kind of chemical action. If one or more of the ingredients 

 be in a state approaching to chemical solution, a similar segregation will often 

 take place ; but the separation of parts will in such case be more complete, 

 and the effect more perfect. I subjoin a few examples of these kinds of se- 

 gregation. 



(1.) Chalk Flints. — The position of nodular flints in chalk is two well 

 known to be here described. I merely observe, in this place, that they are, in 

 their origin, unquestionably posterior to the beds in which we find them. What 

 caused them to aggregate on the very points where we find them is not the 

 question. As a matter of fact the free siliceous matter of the chalk formations 

 is not distributed uniformly, but has been accumulated in distinct masses 

 and concretions, and therefore exemplifies the principle for which I am con- 

 tending. 



(2.) Globular Calciferous Grit, ^c. — Calciferous grit has very often a dull 

 fracture, and the presence of carbonate of lime can hardly be detected except 

 by tlie use of an acid. In short, we have an irregular mechanical aggregate 

 of earthy parts, and no definite separation produced by crystalline action. 



In the next modification of calciferous grit, we always obtain, on fracture, 

 a series of ghstening surfaces. The peculiar chatoyant lustre of such speci- 

 mens arises from this cause — that the calcareous matter has crystallized through 

 certain definite spaces in systems of parallel plates. Hence, in turning such 

 specimens about, we first have a reflexion of light from one system, then a 

 duU surface, and then again a bright reflexion for a second system of pa- 

 rallel plates. In this mode of examination we might, I doubt not, sometimes 



