PHYSIOGRAPHIC PROBLEMS OF TO-DAY 639 



into the earth's crust, instead of diastrophic movements, as com- 

 monly supposed. The activity of volcanoes at the present day is 

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

 injection is still in progress, and is to-day producing changes in the 

 geography of the earth's surface. 



Of still more importance to the physiographer than the surface 

 changes known, or legitimately inferred, to have resulted from the 

 formation of dikes, laccoliths, and intruded sheets are the elevations 

 and possibly concurrent depressions of the surface of the lithosphere 

 caused by still greater migrations of portions of the earth's central 

 magma outward and into or beneath the rigid surface rind. Concern- 

 ing these regional intrusions, as they may be termed, the geologist 

 has furnished suggestive information. We are told, for example, 

 that the granitic rocks from which the visible portion of the Bitter 

 Root Mountains in Idaho have been sculptured are intrusive. The 

 now deeply dissected granitic core of this mountain range measures 

 not less than three hundred miles in length and from fifty to over 

 one hundred miles in width. The area occupied by intrusive granitic 

 rocks in the Sierra Nevada is seemingly still greater than in the case 

 just cited, and other regional intrusions of even mightier dimensions 

 are known in the vast region of crystalline rocks in Canada and else- 

 where. The covers of sedimentary or other material which formerly 

 roofed these vast intrusions in the instances now open for study have 

 for the most part been removed by denudation, although instructive 

 remnants of metamorphosed terranes occurring as inliers in the 

 granitic areas sometimes persist and reveal something of the nature 

 of original domes of which they formed a part. 



The surface changes in relief produced by the migration of magmas 

 measuring thousands, and in many instances, as we seem justified in 

 concluding, tens of thousands, of cubic miles, from deep within the 

 earth outward, but failing to reach the surface, must be reckoned 

 as of major physiographic importance. The very magnitude of the 

 features of the earth's surface due to such intrusions has served to 

 conceal their significance. We look in vain in our treatises on physio- 

 graphy for so much as a mention of them. Perhaps the excuse will 

 be offered that the modifications in relief referred to are commonly 

 grouped with the results produced by diastrophic agencies; but, if so 

 considered, a differentiation seems necessary, and the significance of 

 the topographic forms resulting from intrusions of various kinds 

 clearly recognized. 



In our dreamed-of museum of ideal physiographic types, mighty 

 domes raised by regional intrusions, broad uplifts with perhaps 

 sharply defined boundaries, elevated by relatively thin intruded 

 sheets, as well as steep-sided domes with relatively small bases, con- 

 cealing laccoliths, and the still smaller covers arching over plutonic 



