the Earth's Contraction from Cooling. 287 



tainly requires a much higher temperature than metamorphism. 

 The evidence appears to be decisive against the making of the 

 vast undercrust lire-sea by this method. 



On the other hand, as already stated, there are in many coun- 

 tries regions of siliceous trachytes whose ejections may well have 

 come from the local fusion of common granite and the allied 

 schistose rocks, or of sedimentary strata of like composition. 



Mallet's theory presents us with a true cause; but what arc 

 the limits of its action it is very difficult to decide. It relieves 

 the theory of local fire-seas as derivative from the old viscous 

 layer of the chief objection urged against it, that such isolated 

 fire-regions could not long exist surrounded by cooled rock ; 

 for if inadequate to make great undercrust fire-seas one or more 

 thousand miles in length, like the Appalachian, the cause may 

 be sufficient through the generated heat to keep the old fire- 

 seas in prolonged existence. 



2. What are igneous rocks ? — From the preceding discussion 

 we derive an idea of the distinction between eruptive and meta- 

 morphic rocks. Since the larger part of eruptive rocks have 

 come from the infra- Archaean region (either the true crust or 

 the fire-seas within or below it), they are igneous in all their 

 history, and in no sense metamorphosed sediments, whether 

 derived from a second fusion of the rocks where they originated 

 or not. 



Again, the plastic rock-material that may be derived from 

 the fusion or semifusion of the supercrust (that is, of rocks ori- 

 ginally of sedimentary origin) gives rise to " igneous n rocks 

 often not distinguishable from other igneous rocks when it is 

 ejected through fissures far from its place of origin, while crys- 

 talline rocks are simply met amorphic if they remain in their ori- 

 ginal relations to the associated rocks, or nearly so. 



Between these latter igneous rocks and the metamorphic there 

 may be indefinite gradations, as claimed by Hunt. But if our 

 reasonings are right, the greater part of igneous rocks can be 

 proved to have had no such supercrust origin. The argument 

 from the presence of moisture or of hydrous minerals in such 

 rocks in favour of their origin from the fusion of sediments has 

 been shown to be invalid. 



3. Source of the ejecting -force. — When the fractures of the crust 

 giving exit to fissure-eruptions are a direct sequence to a long 

 continued subsidence (as, for example, in the case of the Trias- 

 sico-Jurassic eruptions of the Atlantic border), there can hardly 

 be a doubt that the lateral pressure causing the subsidence con- 

 tributed also to the ejection of the plastic rock from beneath. 

 And as the great fissmings of the crust are in all cases incidents 

 in the working of lateral pressure, it is unsafe to deny that this 



