638 PHYSIOGRAPHY 



and depressions, produced by the migration of portions of the earth's 

 central magma from the deep interior toward or to the surface. A 

 convenient but arbitrary subdivision of the matter forced outward 

 from the earth's interior is in vogue among geologists, and rocks of 

 plutonic and of volcanic origin are recognized. To the physiographer 

 the distinction referred to is more suggestive than it appears from the 

 point of view of the geologist, since the recognition of differences 

 between topographic forms produced by the injection of fluid or 

 plastic magmas into the cooled, rigid outer portion of the earth, and 

 topographic forms resulting from the extrusion of similar matter at 

 the surface, is of genetic significance. 



The simile was used above between the quarry-blocks taken to the 

 studio of the sculptor and the portions of the earth's surface brought 

 by diastrophic movements within the sphere of influence of denuding 

 agencies. There are two other primary classes of physiographic 

 quarry-blocks; one produced by intrusions of highly heated plastic 

 or fluid magmas into the earth's crust, which cause upheavals of the 

 surface above them, and the other due to extrusions of similar 

 material at the surface, as during volcanic eruptions. The first of 

 these two series of earth-features has received much less attention 

 from physiographers than the second series. 



Surface elevations due to local intrusions are well illustrated by 

 the reconstructed forms of the Henry Mountains and the similar 

 information in hand concerning several other regions. The topo- 

 graphic forms referred to have a conspicuous vertical measure in 

 comparison with their breadth of base, and their prominence gained 

 for them earlier recognition than in the case of related, and in part 

 far more important, plutonic changes. It is to be remembered, how- 

 ever, that every intrusion of a magma into the earth's crust is, theor- 

 etically at least, accompanied by a change in the relief of the surface 

 above. What surface changes accompany the lateral movements 

 in the rocks invaded by a dike has eluded search and seemingly 

 escaped conjecture. The surface changes produced by an extensive 

 horizontal injection of a magma, as when intruded sheets are found 

 in stratified terranes, is a matter of inference rather than of observa- 

 tion. Intrusive sheets are numerous, and the surface changes in 

 topography, and consequently of drainage, that accompanied their 

 production must have been important, but definite examples are 

 wanting. Critical studies are needed in this connection, both by 

 physiographers and by geologists, in order that the widely extended 

 movements which have been observed in the surface of the litho- 

 sphere may be referred to their proper cause. How do we know, 

 for example, that the many recorded changes in the relation of the 

 land to sea-level may not in part be due to the injection of magmas 



