are typical gneisses which may resemble igneous rocks; the schists
of later origin exhibit a progressive transition to normal sediments.
Without admitting that it is possible to classify metamorphic rocks
according to the depth at which they were situated when metamorphosed,
we may admit that there is much reason to believe that
the more intense stages of alteration characterize as a rule the rock
masses which were oldest or most deeply situated during the epoch
of folding.
While rocks near the surface which are under comparatively slight pressures yield to stress by fracturing, it is conceivable that at greater depths the minerals would become plastic and suffer deformation without rupture. For this zone of “flowage,” as he terms it, van Hise estimates a depth of not more than 12 kilometres, depending on many factors such as the strength of the rocks and nature of the minerals concerned, the temperature, amount of moisture and rapidity of the deformation. Between it and the zone of fracture, which lies above, a gradual transition must take place. Doelter, on the other hand, believes that the depth at which plastic flow begins must be at least 35 kilometres; it is difficult to imagine that rocks which have been so profoundly buried can now be exposed at any part of the earth’s surface.
In the attempt to explain the existence of large masses of metamorphic rocks which are perfectly foliated, but at the same time coarsely crystalline, and show no grinding down of their components, as might be expected on the hypothesis of pure dynamo-metamorphism, F. Becke brought into prominence another principle which may prove to be widely applicable. Although known as Riecke’s law, it was advanced many years ago by Sorby. It enunciates that when minerals are subjected to unilateral pressure (acting in a definite direction and not like hydrostatic pressure, equally in all directions) they tend to be dissolved on those sides which face the pressure, while the sides which are not compressed tend to grow by additional deposit. Minerals having platy or rod-like forms will thus be produced, all having a parallel orientation, and the rock will be schistose, with foliation corresponding in direction to the extension of the mineral plates, and perpendicular to the stresses which were in action. The solvents which dissolve the mineral on one side and deposit it on the other side are the interstitial moisture and vapours present in the rock. By this means schists and gneisses will be produced, which are perfectly foliated yet have their minerals homogeneous and uncrushed. Experimental data are at present wanting to show how far this principle is operative and what are its limits, but as a supplementary contribution to the theory of dynamo-metamorphism it may prove to be of great importance. This has been described as the development of “schistosity by crystallization.”
More interesting still are E. Weinschenk’s theories of pressure-crystallization and piezo-crystallization (pressure-contact action). He adduces evidence to show that many gneisses are igneous rocks which were foliated from the first, and a large body of observations in many European countries confirms his statement. In his opinion plutonic rocks crystallizing under certain conditions of pressure necessarily assume a banded structure, and contain minerals which are not identical with those of igneous rocks but with the components of schists and gneisses. In the surrounding rocks there is contact alteration but not of the ordinary type as the recrystallized products also have a banding or foliation owing to the pressure acting on them during metamorphism. Bonney urged the hypothesis that many gneisses are merely plutonic igneous rocks which exhibit a flow banding and an imperfect idiomorphism of their minerals owing to their having been injected in a half-solid state; the component crystals by mutual attrition assume rounded or lenticular forms. Undoubtedly there is much truth in these hypotheses, yet in both cases they seem to necessitate the presence of extraordinary earth-pressures such as accompany mountain building. We know that heat greatly increases the plasticity of rocks. Assuming that intrusions take place during an epoch of earth movement, we may be certain that as solidification goes on the pressures will force the rock forward, and the structures will be very different from those assumed by a rock which has crystallized in a condition of rest.
Lastly, there are many geologists who hold that certain kinds of gneiss are due to the injection of plutonic igneous rocks as masses of all sizes into sedimentary schists forming a mélange. The igneous rock veins the sediment in every direction; the veins are often exceedingly thin and nearly parallel or branch again and again. In this way a banding or foliation is set up, and the mixed rock has the appearance of a gneiss. In the sediment, intensely heated, new minerals are set up. The igneous rock digests or absorbs the materials which it, penetrates; and it is often impossible to say what is igneous and what is sedimentary. Acid intrusions may in this way break up and partly assimilate older basic rocks. Very good examples of this process are known, and they may be much more common than is at present suspected. Conditions which favour assimilation at great depths are the enormous pressures and the high temperature of the earth’s crust; the igneous rocks may also be much above their consolidation points. It is quite reasonable to believe that at deep levels absorption of sediments by igneous masses goes on extensively, while in higher, zones there is little or none of this action. (J. S. F.)
METAMORPHOSIS, a term used in zoology in different
senses by different authors, and sometimes in different senses
by the same author. E. Korschelt and K. Heider, in their
work on the development of the Invertebrata, usually apply it
to the whole of the larval development. For instance, in their
account of the Bryozoa, they say (p. 18, part 2, of the English
translation): “The metamorphosis of a Bryozoan larva comprises
a more or less protracted free-swimming stage during which
no perceptible advance is made in the development of the
larva, and the subsequent somewhat complicated changes
which bring about its transformation into the first primary
zooid of the young Bryozoan colony.” Throughout their
account of the Crustacea they use the word in the same sense,
i.e. as applied to the whole of the changes which the larva
undergoes in passing into the adult. On the other hand,
in their account, of Mollusca they seem to restrict the term to
the final change by which the larva passes into the adult form
(op. cit., part 4, p. 14). F. Balfour in his great work on Comparative
Embryology seems to limit the word to a. sudden change
in the larval history. For instance, he says: “The chief point
of interest in the above development is the fact of the primitive
nauplius form becoming gradually converted without any
special metamorphosis into the adult condition” (Comparative
Embryology, 1885, i. 463). “By the free Cypris stage into
which the larva next passes a very complete metamorphosis
has been effected” (op. cit. i. 490). “The change undergone
by the Tadpole in its passage into the Frog is so considerable
as to deserve the name of a metamorphosis”
(op. cit. ii. 137). Finally and most decisively he says in
his general account of larvae: “In the larval type [of development]
they are born at an earlier stage of development, in a
condition differing to a greater or less extent from the adult,
and reach the adult state either by a series of small steps or
by a more or less considerable metamorphosis” (op. cit. ii. 360).
Here the term will be used in the sense of the last quotations
from Balfour and will be regarded as applicable only
to those cases of sudden and marked change which frequently
occur at the end of the larval period and sometimes
at more or less frequent intervals during its course (Crustacea).
Some authors (see H. G. Bronn, Thierreich, “Myriapoda,” Bd. 5, Abth. 2, p. 113) have applied the term “metamorphosis” only to those cases of larval development in which the young leaves the egg with provisional organs which are lost in the later development. Such authors apply the term “anamorphosis” to cases in which the just-hatched young is without provisional organs but differs from the adult in size, and in the number of segments and joints, &c. Such writers apply the term “epimorphosis” when there is merely an acquisition of sexual maturity and increase in size after birth or hatching.
The essential feature of metamorphosis is the sudden bursting into function of new organs, whether these organs suddenly arise or have been gradually formed, without becoming functional in preceding larval stages. Another feature of it is the disappearance of organs which have been of use to the larva but which are not required at all or are not required in the same form in the new environment. The term is only used in connexion with larval development and is not applied to the sudden changes, due to a change of environment (e.g. the passage of the mammalian embryo from the oviduct into the uterus), which sometimes occur in embryos. Neither is it used in Connexion with the sudden changes of conditions which occur at the birth or hatching of an embryo, although, especially in the case of birth, this event is frequently accompanied by profound morphological alteration.
The most familiar examples of metamorphosis are the abrupt changes which occur at the end of the larval history of the frog and of many insects. In both these cases there is a sudden and great change of environment; there is a sudden demand for new organs which would have been quite useless in the old environment, and organs which were of use in the, old environment and are of no use in the new have to be eliminated. The two examples we have chosen have the advantage of showing us the two methods by which the crisis in the life-history is met.
