120 ORIGIN AND DEVELOPMENT OE ORE DEPOSITS. 



Columnar jointing. The gradual conversion of fine mud into 

 more or less hardened shale, which can be observed in any dried- 

 up pond, is the simplest example of the production of jointing. 

 By this drying, not only is the mud hardened, but joint-structures, 

 somewhat similar to the columnar structures so characteristic of 

 basalt, are often produced. At the same time a kind of 

 lamination is developed, which is due to the existence of minute 

 differences in the sediments not observable while the mass was 

 still soft and wet. The mud, as it falls to the bottom, forms at 

 first an apparently homogeneous mass ; but as soon as it begins 

 to dry, especially if there is any considerable thickness of it, 

 slightly differing, and separable layers begin to be evident. 



We may sometimes see these induration structures very well 

 developed in the clay filling the old-fashioned shallow "clay-pans" 

 of the china clay works. The short columns produced by natural 

 drying are very different in size, and the polygons are very 

 irregular, the number of sides varying from 3 up to 7 or 8, 5 or 

 6 being the most common number. The lamination is rarely 

 very distinct, unless the clay has been "badly washed" — 

 imperfectly freed from mica, — but in such cases it becomes very 

 evident when the clay is thoroughly dry, although not visible 

 while it is moist. It is surprising how hard some of these clays 

 become when the drying happens to be very slow.* 



Columnar jointing, produced by solidification from a state 

 of fusion, has its best development in basalt, where it is evidently 

 due to contraction and condensation around certain centres during 

 solidification, as appears from the well-known experiments of 

 Gregory, Watt, and others. 



A similar jointing, due to the action of heat not sufficient to 

 produce fusion, was produced in the experiments of Professor 

 Chandler Eoberts- Austen. Having heated a mixture of clay 

 and sand very strongly (1020° C), and allowed it to cool slowly, 

 he found it contracted 6 per cent., having also assumed a well- 



*Mauy of the substances found in Cornish mines are quite soft when first met 

 with, but harden considerably on exposure ; as for instance Pitticite, the beautiful 

 banded lithomarge found in the mines of Redruth, Illoeran, and Camborne, and 

 the semi-opal of St. Just. This is also said to be the case with Chalcedony in 

 many localities. In some cases this hardening may be due to simple evaporation, 

 in others it probably indicates a molecular change in the mass. 



