70 C'. G. Gnic'lin vn Clinkstone or PItonolite. 



in that of the PJ'erdekuppe, the specific gravity is ^.605 ; the 

 fresh clinkstone of Abstrode equal 2.623 ; the decayed, from 

 jthe same place, which has parted with nearly all its mesotypic- 

 matter, is 2.651. 



4. When a clinkstone affords a small quantity of silica, and 

 a larger quantity of alumina, we infer a considerable intermix- 

 ture of mesotype : if the silica is above 60 per cent., the clink- 

 stone is nearly a pure felspar. The analysis of Klaproth, Berg- 

 man, and Dr Struve, point out clinkstones that contain less me- 

 sotype than the clinkstone of the Hegau, but more than that of 

 the Rhongebirge. Dr Struve analyzed the white decayed crust, 

 and also the fresh mass under it, of the clinkstone from the Ro- 

 thenberg near to Brux, from which it results, that, by the pro- 

 cess of decomposition, the clinkstone is deprived of its mesotype, 

 and is converted into a pure felspar mass *. 



.'5. This composition of clinkstone excludes the idea of the 

 formation of the zeolitic crystallization, by a process of injiltra- 

 Uon : it is evident that the very generally distributed natrolite 

 in the cone of Hohentwtel, has separated itself by a process of 

 secretion from the mass of the rock in which it is contained. 

 Natrolite occurs in but small quantity in the cone of Hohen- 

 krahen, probably because the lava on cooling, split into fewer 

 rents, and thus afforded fewer opportunities for the secretion of 

 crystals. 



6. Although clinkstone and basalt have been formed in the 

 same way, they present a remarkable contrast, which we shall 

 now notice. In clinkstone, natron, potash, and silica prevail ; 

 whereas in basalt, oxide of iron, magnesia, and lime, are the 

 predominating constituent parts, natron and potash being nearly 

 excluded. We can say, natron and potash characterize clink- 

 stone, iron, and magnesia basalt. This is remarkably the case 

 with the clinkstones and basalts of Hegau : The first contain no 



• Dr Struve considers, as worth}' of notice, the fact ascertained by his re- 

 searches, viz. that, in the decayed minerals, the proportion of potash was 

 nearly double that in the undecaj'ed, whereas the quantity of soda diminished. 

 It may be explained by remarking, that the zeolitic constituent part is re- 

 moved by the act of weathering, while the felspar remains. In the first, 

 natron predominates, in the second, potash prevails ; hence, in the decayed mi- 

 neral the relative quantity of potash will increase, and the absolute quantity 

 of alkaline matter diminish. 



