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



Fine examples of the structure here described may be seen in the hills south 

 of Conway, on the north side of the great road a mile or two east of Bettws 

 y Coed, in the rugged hills east of Beddgelert, as well as in many other parts 

 of the great Welsh chain. 



(5.) Nodular Ironstone, Septaria, ^c., inheds of Shale. — These phenomena 

 are so well described in the fifth chapter of Mr. De la Beche's " Theoretical 

 Researches/' that I have little or nothing to add to what he there states ; but 

 I may quote him in this place as an authority for the conclusions 1 am endea- 

 vouring to support. lie describes calcareous nodules in the lias of Lyme 

 Regis, and tells us "that on fracture they are found of a laminated structure, 

 and that the laminae of the nodules are precisely parallel to the laminas of the 

 shale, or marl, in which they are inclosed, and little doubt can exist that they 

 once constituted continuous portions of each other," He considers this struc- 

 ture "as the result of the attraction of certain particles among each other, 

 after their deposition ;" in which conclusion it is hardly necessary for me to 

 state that I entirely concur with him. Again, he says, " the ironstone no- 

 dules of the coal measures seem to have been thus produced." It is true that 

 in such nodules the laminations are seldom visible, and the process of gradual 

 segregation is, therefore, less obvious. But I possess some ironstone nodules 

 from Yorkshire which are perfectly laminated ; their origin and mode of se- 

 gregation can, therefore, admit of no doubt. 



Nodular concretions (like those just described), in beds of shale, though 

 very imperfect as mineral phaenomena, are very instructive; in as much as they 

 often point out the causes which led to the aggregation of the nodules on the 

 very spots where we find them. The first segregation appears often to have 

 been caused by the presence of some extraneous substance, some fragment 

 of an animal or a vegetable, or, perhaps, some minute and almost invisible 

 granule. This is in exact accordance with what we know experimentally of 

 crystallization and precipitation. Whatever be the health of the patient, if a 

 nucleus be once formed in the bladder, the calculus will go on increasing ; 

 and there are numberless instances of the effects of a like principle. The 

 j)articles in a menstruum may, perliaps, be regarded as in a kind of unstable 

 equilibrium, which is disturbed by the presence of any extraneous body. At 

 all events, as a matter of fact, extraneous substances promote crystalline pre- 

 cipitation ; and, therefore, in the great operations of nature, form the centres 

 of concretions segregated from stratified beds. But what cause was it which 

 first determined the chemical nature of the materials forming the nucleus of a 

 nodular concretion, or the lapidifying substance of a petrifaction ? If like 

 things tend to aggregate with like, we have a principle which explains, in 



