182 



of the latter class have often been effected during indefinite periods of 

 time, and under circumstances which admit not of imitation. In such 

 cases it is by observation only, and not by direct experiment, that we 

 can hope to rise to a rational explanation of the phaenomena. The 

 author then gives some examples of both kinds of change here con- 

 sidered. 



§ 2. Globular Concretionary Structure. 



The author remarks, that although this kind of structure, as seen 

 in aqueous rocks, can seldom be imitated, yet it may be explained, in 

 most cases, compatibly with the known modes of material action, and 

 the phsenomena may be correctly classified. He then proceeds to 

 give examples of the structure in question. 



1. Chalk Flints. — ^They are posterior to the existence of the beds 

 in which they are found. The free siliceous matter of the formation 

 has not been uniformly diffused, but accumulated in distinct concre- 

 tions ; and therefore illustrates the principle contended for in the 

 paper. 



2. Globular Calciferous Grit, &;c. — The author dwells at consider- 

 able length on the internal structure of calciferous grits, and explains 

 their chatoyant lustre by the independent crystallization of carbonate 

 of lime through definite spaces. He then points out several cases of 

 such rocks with a regular spheroidal structure, and vi^ith the lamina- 

 tions of original deposit passing, without interruption, through the 

 several spheroids ; and he infers from such pheenomena that the glo- 

 bular structure was superinduced during the passage of the stratified 

 mass into a solid state. 



3. Globular Magnesian Limestone. — For a detailed account of this 

 structure, he refers to a former memoir in the Society's Transactions, 

 and endeavours to confirm, by new arguments, the conclusion he 

 drew from the phsenomena, viz., that all the complicated concretions 

 in the formation of the magnesian limestone, have been produced since 

 the original deposition of the beds*. 



4. Rocks of Globular Structure subordinate to the Old Slate Forma- 

 tions of North Wales. — He describes these rocks in considerable de- 

 tail, and divides them into two classes, both of which he is disposed 

 to arrange among stratified rocks altered by igneous action ; and he 

 remarks, that whether this opinion be true or false, the phsenomena 

 illustrate a great principle in the segregation of mineral masses. 



5. Nodular Ironstone, Sfc. in Beds of Shale. — These, again, are pos- 

 terior to the deposition of the beds ; for it is shown (especially by 

 some examples derived from Yorkshire) that the laminations of de- 

 posit may be traced through the nodules themselves. In this case the 

 segregation of the nodule has often been occasioned by the presence 

 of an extraneous body. Other examples are given of a similar che- 

 mical segregation from a similar cause ; and the section concludes 

 with an enumeration of some appearances exhibited in the mineral 

 structure of petrifactions. 



* See Geological Transactions, Second Series, vol. iii. p. 94, et seq. 



