BY THE CONDITIONS OF THEIR EXTRUSION. 15 



that at which dyke stones will melt. The very fact that they are deposited against lavas, 

 proves that they can be can-ied at a lower temperature than that at which the dykes stones 

 can be melted. 



It remains to be noticed that dykes do not generally show much evidence of gas 

 action. We cannot regard this element of force as the principal power at work in their 

 injection. "Were it a very generally efficient cause, our dykes should show us the blebs 

 or gas centres which are so common in volcano lavas. These are manifestly wanting in 

 the greater part of our included lavas. 



To sum up the most important part of these propositions concerning included lavas in a 

 few words, we may say that there is good reason for suspecting that they are in many cases 

 indigenous in one particular series of metamorphic rocks ; that they are derived from the 

 most easily molten parts of the section, i. e., the very siliceous beds; that they may extend 

 upwards or downwards from tlieir point of origin ; that they are in the main im- 

 pelled into the position where they are now found by a combination of the pressure of 

 the superjacent beds and the formation of a vacuum by the opening of a fissure ; and, 

 finally, that they difler from volcanic ejections in the absence of a powerful escape of 

 gas, which in the volcanic outbreak is the agent that forces the ejected materials to the 

 surface. 



