PHYSIOGRAPHIC PROBLEMS OF TOD/IY 537. 



topographic forms resulting from the extrusion of similar matter at 

 the surface, is of genetic significance. 



The simile was used above between the cjuarry 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 physi- 

 ographers 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 earher recognition than in the case of related, and in part far 

 more important, plutonic changes. It is to be remembered, however, 

 that every intrusion of a magma into the earth's crust is, theoretically 

 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 observation. 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 lithosphere 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 into the earth's crust, instead of diastrophic movements 

 as commonly supposed. The activity of volcanoes at the present day 

 is warrant for the hypothesis that the concurrent process of sub- 



