Relations in New England between batholithic and metamorphic limits. The pre-Newark floor of the Connecticut Triassic. The section across southwestern Connecticut. The section across northwestern Connecticut. Lack of metamorphism in Pennsylvania folds. PART II. Metamorphic and metasomatic relations of orogenic batholiths. Heating and crystallizing effects of magmas. Chemical or metasomatic effects of emanations. Prevention of deep penetration of meteoric waters. Development of anhydrous or hydrous silicates. Silication in relation to igneous intrusion. Relation of magmatic gases to atmosphere and ocean. Limited depth of zone of anamorphism and rock flow. PART III. Interpretation of dynamo-metamorphic features in the roofs of Features produced by movement of solutions and selective crystallization. Development of lit-par-lit structure by the force of crystallization. Development of banded orthogneisses as a result of successive injections. [In summary, evidence is presented (Part I) that 1 This paper was written by Professor Barrell apparently in the year 1913 AM. JOUR. SCI.-FIFTH SERIES, VOL. I, No. 1.-JANUARY, 1921. Batholiths like those in the American Cordillera seem to come to place without crustal compression, but those of the Archean shield and those of the later Appalachian invasions are accompanied by compression. A detailed study of three or four regions shows the metamorphism to be related to the igneous invasion more than to the depth and pressure. One of the regions of deepest burial and close folding in Pennsylvania shows slight metamorphism. II. The action of magmas, both by heating and metasomatism, is reviewed. The solutions are not meteoric in origin. The results in minerals depend on equilibria,largely on the presence of H2O and CO2. The depth of anamorphism may be small, due (1) to weakness of some rocks, (2) to invasion of batholiths. An argument for shallow depth is based on the completeness of Archean metamorphism and the salt of the ocean as a measure of erosion. III. The features of metamorphic rocks are reviewed and interpreted as due to one or another factor. Major factors are batholithic invasion and compression. Movements of solutions, selective crystallization, lit-par-lit injection gneisses, and the alternation of injection and mashing, each leave their marks.] PART I. REGIONAL RELATIONS. INTRODUCTION. The phenomena of regional metamorphism are manifested in a widespread crystallization and reconstruction of rocks, in granulation and recrystallization. Mashing and shearing compete with folding, as three distinct modes of yielding, in giving complicated structures; mashing and recrystallization lead to segregation of minerals in layers, and are manifested in a dominant foliated and banded arrangement which formerly was taken as evidence of sedimentary origin. Metamorphosed terranes pass in places into massive holocrystalline rocks which are now proved in most cases to be of intrusive igneous origin. In rare cases, however, they seem to be completely recrystallized sediments. A former interpretation which prevailed in this country until near the close of the nineteenth century regarded all these gneisses, including the granite ones, as merely the last term of metamorphism of sediments in place, especially in the heated and softened bottoms of geosynclines. Orogenic deformation was looked upon in connection with the trapped sea waters as the agent which accomplished the work. The newer interpretations, however, regard the fluids and gases of igneous rocks as of juvenile origin, making their way for the first time toward the surface of the earth. These present views in regard to the origin of the granitic rocks and their emanations raise the problem as to how far igneous action is essential for the development of regional metamorphism. To what extent does regional metamorphism imply the invasion of deepseated bodies of magma of regional extent, even when these give no direct evidence of their presence; or to what extent may mere crustal compression and deformation of deeply buried rocks carry forward this work? Since, by the very terms of this question, the magmatic invasions, if they are the controlling causes of regional metamorphism, are concealed, the problem can not be directly solved. It must be attacked by inference, but the attack is none the less desirable, since it raises problems with many bearings, and directs the attention to the observation of field relations. The accumulation of these from many sources and by many minds must in the end give adjusted quantitative value to the opposing factors and lead to a better understanding of the mysterious inner earth. The problem and the conclusions, as outlined in this article, were formulated in the mind of the writer from 1904 to 1906, and presented more or less completely to successive classes of advanced students. The points for first presentation are profound metamorphism of some large areas in New England where igneous rocks are near by, but the depth of burial slight; and the relatively small effects in a Pennsylvania region where igneous rocks are absent, but the depth of burial has been very great, and folding fully as close as in the New England region.2 2 The fact that in all the intensely metamorphosed * [The degree or intensity of metamorphism is judged by the usual criteria: deformation of original structures and textures; formation of new minerals; recrystallization; shape, size, and arrangement of grains with resulting cleavage and luster; and changes in the composition of the rock. The changes that occur without notable additions were discussed by Lahee in a paper on the crystalloblastic order and mineral development in metamorphism (Jour. Geology, 22, 500, 1914), but in this paper instances are cited where the changes are more profound and result from magmatic additions.] province of the southern Appalachians no post-Cambrian intrusives were mapped by the geologists who worked in that field, whereas much of the metamorphism was of post-Cambrian date, led the writer to hold the matter in considerable reserve, and refrain from publication. But in 1907 Keith in the Pisgah, North Carolina, folio3 noted the undetermined age of the Whiteside granite which occupies large areas in North Carolina and Georgia, and stated that it might be as late as Carboniferous. In the geological map of North America issued in 1911, large granite areas in the southern Appalachians are mapped as post-Cambrian; and in the Ellijay, Georgia, folio,* 1913, La Forge and Phalen state, in regard to the small patches of younger granite on the east and the gabbro dikes cutting the Lower Cambrian on the west, that from their structural relations both are believed to be of late Paleozoic age. This increase of knowledge brings the late Paleozoic orogenic history of the southern Appalachians into closer kinship with the late Paleozoic history of New England. It was the study of the latter field, in addition to some years of experience in the Cordilleran province, which led to the development of the ideas expressed in this article. They apparently may be extended to cover the southern Appalachians, and seem to imply enough generality of application to warrant publication. Various additions have been made to utilize the contributions bearing on this subject made by others in later years. SUBJACENT IGNEOUS INVASION IN MOUNTAIN PROVINCES. Suess and Daly have in separate volumes broadly discussed in recent years the phenomena of batholithic invasion. Adams and Barlow, Sederholm, and others have made clear many detailed illustrations. The cumulative effect of all this work is to show the widespread occurrence of those bodies named batholiths by Suess, which he, in common with the French geologists, held to broaden downward "bis in die ewige Teufe", and which Daly classifies as subjacent igneous bodies in order to distinguish them from those clearly injected into the crust. In broad regions, these are now known to be of Paleozoic, Mesozoic, and even of Cenozoic age, extending A. Keith, U. S. Geol. Survey, Geol. Folio 147. *L. LaForge and W. C. Phalen, U. S. Geol. Survey, Geol. Folio 187. through geological time that process which operated so widely in the Archeozoic and destroyed by batholithic invasion, perhaps everywhere, the older foundations of the earth. Harker makes the valuable distinction between the character of igneous intrusion in plateau and in mountain provinces. In the first, the intrusions have not been connected with great compressive movements; in the second, they are so related, and this determines a different habit in the forms of the intrusive bodies. Harker, however, does not go into the problem of the granite gneisses and their contacts as exhibited in provinces marked by regional metamorphism. In fact, he favors Brogger's view that the granite batholiths are nothing more than large and irregular laccoliths. In the classification of batholiths according to the environing conditions which existed at the time of their solidification, we may draw two distinctions. First, they may have approached to within a few thousand feet of the surface of the earth, into a zone of fracture and rapid cooling, or they may have ceased their upward progress at a depth of miles, while still in the zone of flow and where their heat and their emanations would persist and affect the roof above them for at least a geological period. Those coming to rest at a depth of a few miles may become exposed in later ages by profound erosion. Those which lie deeper, if there be such, can never be recognized, except indirectly by an interpretation of their effects. Secondly, we may divide batholiths, following the suggestion of Harker, into those whose rise has not been accompanied by regional deformation, and, on the other hand, those during whose rise great regional deformation has prevailed. To illustrate these categories: The Boulder batholith of Montana is a body whose exposed portion covers about 1100 square miles, and whose cover over broad areas consisted merely of a somewhat earlier series of extrusive andesites a few thousand feet in thickness. Some folding of the older sedimentary rocks seems to have gone on before the invasion of the batholith, but the latter shows no effects of compression. The disturbances at the time of its origin and later seem to have been essentially movements of vertical adjustment. 5 In New England, on the other hand, the granites are * A. Harker, Natural History of Igneous Rocks, Chap. III, 1909. |