538 ISRAEL C. RUSSELL 



surface injection is still in progress, and is today 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. Con- 

 cerning 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 physiog- 

 raphy 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 lands 

 clearly recognized. 



