15 



I will attempt briefly to put it thus : — 



Igneous rocks are being poured forth from a volcanic vent, in 

 perfectly fluid or at least plastic flow ; some are dense, some 

 scoriaceous, some frothing, and so when solidified are vesicular, 

 or perchance even hold in suspension bubbles of included water, 

 this latter holding in solution (red-hot solution) solids after- 

 wards to separate as rheolites. Should the air-bubbles of the 

 vesicular rocks arise through the plastic mass while it is motion- 

 less, these bubbles will be more or less rounded or pear-shaped. 

 Should the solidifying rock, however, become crystalline or 

 porphyritic, as generally is the case with amygdaloids, the sepa- 

 rating crystals of labradorite, &c., will more or less roughen the 

 sides, and so destroy the smooth and rounded figure of the 

 cavity ; while, if the lava-flow continues its motion while the 

 bubbles are still rising, their shape will be more or less flattened 

 or altered : — try bubbles in flowing treacle. 



Stage the first. — An empty cavity of any shape. 



Stage the second. — The rock, while solidifying, may contain 

 an excess of a magnesian mineral, which is exuded into the 

 cavity ; or this excess of magnesian compound (magnesia not 

 being, to any large extent, a natural constituent of the mass of 

 a trap) may be held as vapour in the cavity, to be, on cooling, 

 deposited on its sides. This forms in Scotland, Faroe, Iceland, 

 &c, the layer of celadonite or delessite ; at Giants' Causeway, 

 of chlorophceite, which, on the extraction of the afterwards 

 filled-up cavity, forms the " skin of the pebble." 



Stage the third. — One of two processes, the first very doubtful. 



The cooling and shrinking rock holds in a state of liquidity, 

 from heat, an excess of colloidal silica, which is exuded into 

 the cavity, forming a chalcedonic druse. But, admitting the 

 process, it must here stop, and a solid agate could not thus 

 be formed. This seems to have been the view of Sir George 

 Mackenzie. 



The other process I pin my faith to. The thoroughly solidi- 

 fied — indeed the now old — rock is having its felspar (labradorite 

 or other) decomposed by water holding carbonic acid in solution. 

 I have proved that this process is rapid and continuous, and 



