Rise and Decline of the Metamorphic Theory of Granite
Up to 1860 granite was regarded on the basis of the facts of the field as essentially an intrusive rock, but gneiss as a metamorphic product mostly of sedimentary origin. It seemed as though sound methods of research and interpretation were securely established. Nevertheless, a new era of speculation and a modified Wernerism arose at that time with a paper by T. Sterry Hunt, marking a retrogression in the theory of granite which lasted until his death in 1892.
In November, 1859, Hunt read before the Geological Society of London a paper on “Some Points in Chemical Geology” in which he announced that igneous rocks are in all cases simply fused and displaced sediments, the fusion taking place by the rise of the earth’s internal heat into deeply buried and water-soaked masses of sediments (see 30, 133, 1860). The germ of this idea of aqueo-igneous fusion was far older, due to Babbage and John Herschel, neither of them geologists, but such sweeping extensions of it had never before been published. Hunt had the advantage of a wide acquaintanceship with geological literature and chemistry. He wrote plausibly on chemical and theoretical geology, but his views were not controlled by careful field observations. In fact he wrote confidently on regions which apparently he had never seen and where a limited amount of field work would have shown him to have been fundamentally in error. A man of egotistical temperament, he sought to establish priority for himself in many subjects and in order to cover the field made many poorly founded assertions. Building on to another Wernerian idea, he held that many metamorphic minerals had a chronologic value comparable to fossils—staurolite for example indicating a pre-Silurian age—and on this basis divided the crystalline rocks into five series. Although there is much of value buried in Hunt’s work it is difficult to disentangle it, with the result that his writings were a disservice to the science of geology. Although carrying much weight in his lifetime, they have passed with his death nearly into oblivion.
Marcou, with a limited knowledge of American geology, and but little respect for the opinions of others, had published a geologic map of the United States containing gross errors. In support of his views he read in November, 1861, a paper on the Taconic and Lower Silurian Rocks of Vermont and Canada. In the following year he was severely reviewed by “T,” who states positively in controverting Marcou (33, 282, 283, 1862) that “the granites (of the Green Mountains) are evidently strata altered in place.”
“Mr. Marcou should further be informed that the granites of the Alpine summits, instead of being, as was once supposed, eruptive rocks, are now known to be altered strata of newer Secondary and Tertiary age. A simple structure holds good in the British Islands, where as Sir Roderick Murchison has shown in his recent Geological map of Scotland, Ben Nevis and Ben Lawers are found to be composed of higher strata, lying in synclinals. This great law of mountain structure would alone lead us to suppose that the gneiss of the Green mountains, instead of being at the base, is really at the summit of the series....
We cannot here stop to discuss Mr. Marcou’s remark about ‘the unstratified and oldest crystalline rocks of the White mountains’ which he places beneath the lower Taconic series. Mr. Lesley has shown that these granites are stratified, and with Mr. Hunt, regards them as of Devonian Age. (This Journal, vol. 31, p. 403.) Mr. Marcou has come among us with notions of mountains upheaved by intrusive granites, and similar antiquated traditions, now, happily for science, well nigh forgotten.”
It is seen that Marcou, notwithstanding the general character of his work, happened to be nearer right in some matters than were his critics, and that “T” had adopted to the limit the views of Hunt.
The recovery of geology from this period of confusion was partly owing to the slow accumulation of opposed facts; especially to a recognition of the fact that the overplaced relation of the granite gneisses in western Scotland was due to great overthrusts; also to the evidence of the clearly intrusive nature of many of the Cordilleran granites. The recovery of a sounder theory was hastened, however, by the application of criticisms by J. D. Dana in the Journal. In 1866 (42, 252) Dana pointed out that sedimentary rocks in Pennsylvania, in Nova Scotia, and other regions which had been buried to a depth of at least 16,000 feet are not metamorphic. Mere depth of burial of sediments was not sufficient therefore to produce metamorphism and aqueo-igneous fusion. The baseless and speculative character of the use of minerals as an index of age and of Hunt’s interpretation of New England geology in general was shown by Dana in 1872 (3, 91). The following year Dana pointed out clearly that igneous eruptions in general have been derived from a deep-seated source and did not come from the aqueo-igneous fusion of sediments. As to gradations between true igneous rocks and fused and displaced sediments he makes the following statements (6, 114, 1873):
“Again, the plastic rock-material that may be derived from the fusion or semifusion of the supercrust, (that is, of rocks originally of sedimentary origin,) gives rise to “igneous” rocks often not distinguishable from other igneous rocks, when it is ejected through fissures far from its place of origin; while crystalline rocks are simply metamorphic if they remain in their original relations to the associated rocks, or nearly so.
Between these latter igneous rocks and the metamorphic there may be indefinite gradations, as claimed by Hunt. But if our reasonings are right, the great part of igneous rocks can be proved to have had no such supercrust origin. The argument from the presence of moisture or of hydrous minerals in such rocks in favor of their origin from the fusion of sediments has been shown to be invalid.”
The injected marginal rocks and the post-intrusive metamorphism of most of the New England granites has, however, obscured more or less their real igneous nature so that the gradation from metamorphic sediments through igneous gneisses to granites could be read in either direction. These features misled Dana who accepted the prevailing idea of the general metamorphic origin of granite. Dana makes the following statement (6, 164, 1873):
“But Hunt is right in holding that in general granite and syenite (the quartz-bearing syenite) are undoubtedly metamorphic rocks where not vein-formations, as I know from the study of many examples of them in New England; and the veins are results of infiltration through heated moisture from the rocks adjoining some part of the opened fissures they fill.”
Granite, although regarded at this time as the extreme of the metamorphic series and originating from sediments, was looked upon as typically Archean in age, though in some cases younger. Such a doctrine permitted such extreme misinterpretations as that of Clarence King and S. F. Emmons on the nature of the intrusive granite of the Little Cottonwood canyon in the Wahsatch Range. This body cuts across 30,000 feet of Paleozoic rocks and to the careful observer, as later admitted by Emmons, shows clear evidence of its transgressive nature. But at that time it was generally considered that granite mountains were capable of resisting the erosion of all geological time. Consequently it did not seem incredible to King and his associates that here a great granite range of Archean origin had stood up through Paleozoic time until gradual subsidence had permitted it to be buried beneath 30,000 feet of sediments.82
It may seem to the present day reader that such a misinterpretation, doing violence to fundamental geologic knowledge as now recognized, was inexcusable; but in the light of the history of geology as here detailed it is seen to have been the interpretation natural to that time. It is true that a careful examination of the facts of that very field would have proved the post-Paleozoic and intrusive nature of that great granite body now known as the Little Cottonwood batholith, but Emmons has explained the rapid and partial nature of the observations which they were compelled to make in order to keep up to their schedule of progress (16, 139, 1903).
Whitney had found some years earlier that the granites of the Sierra Nevada were igneous rocks intrusive into the Triassic and Jurassic strata. The Lake Superior geologists began to show in the eighties that granite was there an intrusive igneous rock. R. D. Irving and Wadsworth noted these relations. Lawson in 1887 pointed out emphatically (33, 473) that the granites of the Rainy Lake region, although basal, were younger than the schists which lay above them. The granite gneisses he held were of clearly the same igneous origin as the granites and neither gave any field evidence of being fused and displaced sediments. From this time forward the truly igneous nature of granite became increasingly accepted until now the notion of its being made of sedimentary rocks softened and recrystallized by the rise of the isogeotherms through deep burial is as obsolete as the still older doctrine of the Neptunists that granite was laid down as a crystalline precipitate on the floor of the primitive ocean.
The recognition of the truly igneous nature of granites has been followed in the present generation by a series of studies on their structural relations and mode of genesis. A number of important initial articles on various aspects of structure and contact relations have appeared in the Journal, but this sketch of the history of the subject may well stop with the introduction to this modern period.