MACKAY ON MANGANESE DENDRITES. 



XXXIX 



the microscope demonstrated the entire absence of organic structure. 

 The specimen came from the neighborhood of St. Mary's Bay, Digby 



MANGANESE DKNDRITES ON RED SANDSTONE. 

 (Reduced to one-fifth of linear dimensions.) 



county. The st ructure of the flag showed that these Manganese den- 

 drites were originally formed between two close layers of the original 

 flaggy sandstone. He suggested as an explanation of the dendritic form 

 of the manganese deposit, the observed fact that when a thin sheet of 

 liquid holds in solution certain substances, and from any cause the 

 solution is becoming supersaturated, these substances, if they have a 

 tendency to crystalize, are not precipitated uniformly like ordinary 

 sediment. The precipitation commences at a point where the super- 

 saturation begins to develope, which, let it be supposed, in the thin 

 plane of cleavage in the flag, was near the outer margin where the 

 deposit salt first made its appearance. Assuming the crystaline attrac- 

 tive force to operate effectively at a distance of, say, the eighth of an 

 inch, the precipitating material would congregate from that distance to 

 the first point of deposition, leaving a clear space of that extent on each 

 side. And as the supersaturation extended inwards, the point would be 

 extended into a line. But, assuming that the wave of supersaturation 

 2 



