312 ANNUAL OF SCIENTIFIC DlSCOVEUY. 



sorbing the coloring matter, which consisted mostly of oxyd of 

 iron. The workmen kept the pebbles in tubs of water containing the 

 oxyd for a longer or shorter time according to the tint required ; the 

 crystalline .bands remained white, the non-crystalline absorbed the 

 color throughout. Prof. Sullivan remarked that the structure of 

 agate illustrated beautifully the difference between colloids and crys- 

 talloid*. The alkaline silicates, by repose, formed these two classes 

 of bodies, and he had no doubt a similar action had been at work in 

 the formation of agate. 



Formation of Granite. Dr. Percy of the London School of mines, 

 in a recent lecture objected to the assertion of geologists that granitic 

 rocks must have been formed by plutonic agencies ; for, said he. there 

 are certain difficulties which have always been in the way of accepting 

 this view of the subject, difficulties known at all events to those who 

 have been accustomed to make experiments on the fusion of mineral 

 substances at high temperatures. This is especially seen by examin- 

 ing the condition of quartz in granite ; it is always found in the crys- 

 talline condition, and has invariably a specific gravity of 2*6. There 

 is not a single instance known to the contrary. Hence there is reason 

 to believe that the quartz could never have been fused, for the moment 

 silica is fused, no matter in what condition it was previously, a pecul- 

 iar glass-like colloidal mass is produced, having a specific gravity 

 which never exceeds 2*3. Therefore there is good reason to conclude 

 that granite could never have been found under the condition of a 

 high temperature. 



