071 the Genesis of Some Scottish Minerals. 199 



so called. The evidence presented by a large number of 

 specimens shows that when the agate cavity had become 

 quite filled with silica-jelly, and before tliis passed from the 

 coagulated to the solid state, osmotic pressure still continued 

 in operation. Modern investigations have shown that the 

 osmotic pressure from without increases with the density of 

 the solution within the cavity, and that the pressure exerted 

 by the solution within the cavity obeys the same law of 

 expansion under a rise of temperature as gases have long 

 been known to do (Boyle's Law). The increase of internal 

 pressure after the agate cavity had become filled, consequent 

 upon the operation of one or the other of these causes 

 (probably that of continued osmosis), — it is possible 

 that the rise of temperature of the nascent agate, con- 

 sequent upon partial crystallisation of its silica, may have 

 contributed to the same result, — eventually caused one 

 or more ruptures of the walls of the agate. This usually 

 commenced at some point where the " skin " was somewhat 

 thinner than elsewhere, or where the walls of the agate 

 cavity presented a weaker spot than was to be found in 

 other parts. A minute orifice usually resulted ; and, through 

 this minute orifice, the unconsolidated part of the Agate was 

 slowly forced out, like oil colour out of a tube. The layers 

 nearest the Clear Chalcedony were the first to be expelled, and 

 then gradually followed the other layers successively nearer 

 the centre. This curious feature Dr Heddle (to whom we 

 are indebted for an explanation of its true nature) gave the 

 name of the Tube of Escape. There is always more or less 

 dilatation of the tube next the Clear Chalcedony Layer, so 

 that the form of that part of the tube very much resembles 

 that of a Florence flask. The tube itself may always be 

 recognised by the fact that the layers next to it are more or 

 less attenuated, and are also bent outwards in the direction 

 of the external orifice. It is to this curious feature that 

 some of the most striking irregularities of structure seen in 

 agate sections are due. 



In many cases it will be seen that the vacant space left by 

 the expelled silica-jelly has subsequently been refilled by a 

 second generation of agate material. There may even be a 



