208 ANNUAL ADDTIESS BY PKOFESSOR DUNS, H.D., P.R.S.E.. 



homogeneous substances to separate from one another and 

 assume the form of nodules, distinct in themselves, though 

 united in the mass. Thus regarded, concretionary action 

 might be held equal to chemical aggregation. In this case 

 the differentiation of the forms now under this notice — that is 

 their comparative isolation from the mass in which they occur 

 — depends on the presence of elements prone to molecular 

 coalescence, which, while in one sense they withdraw from 

 tlie mass, yet in doing so, carry with them parts of the mass, 

 and thus do not often differ in colour from the mass itself. 

 When the concretion consists mainly of lime they are named 

 calcarcou? ; of silica, siliceous ; of clay, argillaceous or clay- 

 stone concretions. The prevailing element gives the name to 

 the series. 



These fantastic forms have for a long time had the atten- 

 tion of geologists, and for a far longer time the superstitious 

 wonder of the ignorant. Indeed, the attemj)ts to account 

 for their origin and shapes alone constitutes a most attractive 

 chapter in the history of physical geology. The Tophus Indus 

 of Linnasus was a concretion of this sort. David lire of 

 Rutherglen, 1793, first called the attention of British geologists 

 to them. About sixty years ago Alexander Brogniart devoted 

 an able essay to their formation and that of agates — Snr les 

 orbicules siliceux. Twenty years later Ehrenberg published a 

 paper on " The forms assumed by uncrystallized mineral 

 substances called kidneys, Imatru-stones and claystones," all 

 of which he included in the term " Morpholites.^' The Imatru 

 Btones had previously been described as an extinct family ot 

 mollusca. The late President Hitchcock, of Amherst, Massa- 

 chusetts, gives them a prominent place in his great work on 

 the Geology of Vermont. Up to Hitchcock's time little notice 

 had been taken of them in Britain. Since then, however, 

 reference is made to them in most geological handbooks. 

 Some years ago the attention of the late Sir David Brewster 

 was turned to them, and he contributed a paper, " On the 

 fairy stones found in the Elwand water near Melrose," to 

 the Proceedings of the Royal Society of Edinburgh (vol. v.). 

 Sir David thought that they had been formed by the drop- 

 ping of water containing the matter of which they are 

 composed. I agree with Hitchcock on this point when he 

 says, " Nothing can be more absurd than to impute their 

 shapes to the mechanical action of Avater." No doubt they 

 are produced chemically in clay made plastic by Avater and 

 holding in solution the carbonate of lime by which, as clay- 



