Fig. 143—Relation of granite intrusion to schist on the northeastern border of the Vilcapampa batholith near the bridge of Colpani, lower end of the granite Canyon of Torontoy. The sections are from 15 to 25 feet high and represent conditions at different levels along the well-defined contact.
The highly mineralized character of the bordering sedimentary strata, and the presence of numbers of complementary dikes, nearly identical in character to those in the parent granite now exposed by erosion over a broad belt roughly parallel to the contact, supplies a basis for the inference that the granite may underlie the former at a slight depth, or may have had far greater metamorphic effects upon its sedimentary roof than the intruded granite has had upon its sedimentary rim.
The physiographic features of the contact belt are of special interest. No available physiographic interpretation of the topography of a batholith includes a discussion of those topographic and drainage features that are related to the lithologic character of the intruded rock, the manner of its intrusion, or the depth of erosion since intrusion. Yet each one of these factors has a distinct topographic effect. We shall, therefore, turn aside for a moment from the detailed discussion of the Vilcapampa region to an examination of several physiographic principles and then return to the main theme for applications.
It is recognized that igneous intrusions are of many varieties and that even batholithic invasions may take place in rather widely different ways. Highly heated magmas deeply buried beneath the earth’s surface produce maximum contact effects, those nearer the surface may force the strata apart without extreme lithologic alterations of the displaced beds, while through the stoping process a sedimentary cover may be largely absorbed and the magmas may even break forth at the surface as in ordinary vulcanism. If the sedimentary beds have great vertical variation in resistance, in attitude, and in composition, there may be afforded an opportunity for the display of quite different effects at different levels along a given contact, so that a great variety of physical conditions will be passed by the descending levels of erosion. At one place erosion may have exposed only the summit of the batholith, at another the associated dikes and sheets and ramifying branches may be exposed as in the zone of fracture, at a third point the original zone of flowage may be reached with characteristic marginal schistosity, while at still greater depths there may be uncovered a highly metamorphosed rim of resistant sedimentary rock.
The mere enumeration of these variable structural features is sufficient to show how variable we should expect the associated land forms to be. Were the forms of small extent, or had they but slight distinction upon comparison with other erosional effects, they would be of little concern. They are, on the contrary, very extensively developed; they affect large numbers of lofty mountain ranges besides still larger areas of old land masses subjected to extensive and deep erosion, thus laying bare many batholiths long concealed by a thick sedimentary roof.
The differences between intruded and country rock dependent upon these diversified conditions of occurrence are increased or diminished according to the history of the region after batholithic invasion takes place. Regional metamorphism may subsequently induce new structures or minimize the effects of the old. Joint systems may be developed, the planes widely spaced in one group of rocks giving rise to monolithic masses very resistant to the agents of weathering, while those of an adjacent group may be so closely spaced as greatly to hasten the rate of denudation. There may be developed so great a degree of schistosity in one rock as to give rise (with vigorous erosion) to a serrate topography; on the other hand the forms developed on the rocks of a batholith may be massive and coarse-textured.
To these diversifying conditions may be added many others involving a large part of the field of dynamic geology. It will perhaps suffice to mention two others: the stage of erosion and the special features related to climate. If a given intrusion has been accompanied by an important amount of uplift or marginal compression, vigorous erosion may follow, whereupon a chance will be offered for the development of the greatest contrast in the degree of boldness of topographic forms developed upon rocks of unequal resistance. Ultimately these contrasts will diminish in intensity, as in the case of all regional differences of relief, with progress toward the end of the normal cycle of erosion. If peneplanation ensue, only feeble topographic differences may mark the line of contact which was once a prominent topographic feature. With reference to the effects of climate it may be said simply that a granite core of batholithic origin may extend above the snowline or above timber line or into the timbered belt, whereas the invaded rock may occur largely below these levels with obvious differences in both the rate and the kind of erosion affecting the intruded mass.