The borrowed Metamorphism is hardened to a convention and a traditional form, and can be trusted almost inevitably to induce the receptive state in an average audience wherever used.
The greater extent of intrusion and metamorphism amongst these Precambrian rocks than amongst later sediments indicates some differences of conditions in the case of Precambrian and later times.
In some instances, the extreme types of metamorphism which they exhibit are the result of the kind of action usually termed pyrometamorphic as has been shewn by Mr G.
In the zone of metamorphism the influence of heated percolating waters, combined with movements in the rocks, are the principal factors which lead to the concentration of mineral substances.
The rocks forming this earliest known land in the Western Hemisphere consist of crystalline schists, gneisses, and granite, which are considered by some geologists at least as having resulted from the metamorphism of sedimentary beds.
Dynamical and chemical metamorphism may occur at less depth than purely heat metamorphism, and it may be presumed takes place in the axes of mountain ranges, even above sea-level.
An important result ofmetamorphism is the production of new minerals in the rocks acted on.
More than one epoch of metamorphism has perhaps occurred, and the entire record now accessible is exceedingly complicated.
The zone of metamorphism lies between a superior zone where alteration by descending water is dominant, and a lower region where alteration due mainly to heat is in control.
Whether the rocks below the zone of metamorphism are fused or not depends on pressure.
In a similar way the sphere of influence of metamorphism as indicated by the age of the transformed rocks in various regions has in a general way migrated from east to west across the continent.
The principal agencies which take part in contact metamorphism are heat, heated waters, pressure, and perhaps movements within the rocks.
All of these rocks show more or less metamorphism either of a thermal character or due to pressure and crushing.
Regional metamorphism may subsequently induce new structures or minimize the effects of the old.
Nor is the metamorphism of the invaded rock limited to simple intrusion.
In places the metamorphism of the series has been carried further--the shales have become slates and the sandstones have been altered to extremely resistant quartzites.
The Vilcapampa batholith has two highly contrasted borders, whether we consider the degree of metamorphism of the country rock, the definition of the border, or the resulting topographic forms.
He confesses that he knows of "no locality where the geologist may better acquaint himself with the order of superposition of the ancient crystalline rocks of the Highlands, or with the dislocations and metamorphismwhich they have undergone.
It is known to be produced in some cases by the metamorphism of coal, which itself is formed of decomposed vegetal tissues.
Suggest how rocks now in process of metamorphism may sometimes be exposed to view.
Metamorphism is most common among rocks of the earlier geological ages, and most rare among rocks of recent formation.
All gradations between it and phyllite may be traced, and in many cases we may prove it due to the metamorphism of slates and shales.
All these theories agree that the present foliated condition of these rocks is due to the intense metamorphism which they have suffered.
The pre-Cambrian rocks are of very great economic importance, because of their extensive metamorphism and the enormous masses of igneous rock which they involve.
We may contrast with metamorphism the action of external agencies in weathering, which render rocks less coherent by dissolving their soluble parts and breaking down their crystalline grains.
Dynamic action generates heat, but contrary to the popular notion, the heat involved in profound metamorphism is usually secondary, and the dynamic action fundamental.
In the process of metamorphism the rock has been hardened, given a slaty cleavage, and innumerable minute scales of mica have developed to produce a silky luster upon the cleavage faces.
That this pure carbon may have been of organic origin before metamorphism has naturally been conjectured.
As the doctrine of mineral metamorphism is now exciting very general attention, we subjoin a few explanatory observations by the 'New Philos.
Metamorphism of the Loudoun formation is quite general.
The Weverton sandstone has suffered less from metamorphism than any of the sediments.
This final stage is macroscopically nothing more than a siliceous slate or schist, and is barely distinguishable from the end products of similar metamorphism in the more feldspathic schists and the Loudoun sandy slates.
Dynamic metamorphism has produced great rearrangement of the minerals along the eastern side of the Catoctin Belt, and results at times in complete obliteration of the characters of the granite.
The metamorphism of the igneous rocks is regional in nature and has the same increase from west to east as the sediments.
The metamorphism of these rocks does not imply any change of their constituent elements, or interference with their bedded arrangement.
Hunt has discussed very fully their chemical characters and metamorphism in his Chemical and Geological Essays.
Further, they have long been accustomed to regard the so-called Azoic rocks as not only destitute of organic remains, but as being in such a state of metamorphism that these could not have been preserved had they existed.
Anterior to the Cambrian is the great series of the Laurentian, which, owing to the metamorphism to which it has been subjected, has so far yielded but the singular Eozoön.
The Laurentian limestones, in consequence of the metamorphism to which they have been subjected, are so highly crystalline (fig.
Metamorphism may affect rocks of any age, though naturally more prevalent in the older rocks, and to this cause must be set down an irreparable loss of much fossil evidence.
Even a lesser temperature, accompanied by the intense pressure conditions, might well occasion the appearances of thermal metamorphism described by Weinschenk, and for which, otherwise, there is difficulty in accounting.
The appearance of these materials at the surface indicates the removal by denudation and the extreme metamorphism of much sedimentary deposit.
Heated water would appear to have been the agent to which much of the metamorphism which affects the rocky strata must be attributed.
Metamorphism has likewise been produced by the chemical action of percolating water, which frequently dissolves out certain minerals, and replaces these with others having often a very different chemical composition.
How is metamorphism on the large scale supposed to have been induced?
This type of metamorphism has been distinguished by the name marmarosis (Sir A.
Lodovico Sforza went to meet him at Alexandria on the 13th of July, and held a council of war there.
Beatrice had long been dead, her children were in exile, and the Moro was wearing his heart out in lonely captivity within the gloomy prison walls of Loches.
This was the first meeting between these two princes, who were destined to become such bitter enemies in days to come.
Andrea Cossa had already brought me the other four, for which I thank you exceedingly; but I feel that, under the circumstances, I ought not to keep them.
All Lodovico's smooth words and tact were needed to keep the leaders in good humour in these trying circumstances.
But the happy peace of these days was destined to be rudely disturbed.
On the 6th of October the king had recovered sufficiently to leave Asti, and while most of his army marched direct to Piacenza, he himself travelled by Casale and through the dominions of his ally, the young Marquis of Montferrat, to Vigevano.
Completion of the |Metamorphism of Eozoic rocks and arrangements of the Solar System.
This use it may have served, and when its effects had been gradually lost through metamorphism and decay, that second great withdrawal of carbon which took place in the Carboniferous period may have been rendered necessary.
Sometimes extensive metamorphism has so affected them that their original characters, including their organic contents, have been destroyed.
It is among igneous rocks that the progressive stages of metamorphism can be most easily traced.
As the degree of metamorphism increases, the lenticles diminish in size, and the intervening crushed and foliated matrix increases in amount, until at last it may form the entire mass of the rock.
Regional metamorphism has arisen in the heart of mountain-chains, and in any other district where the deformation of the crust has been sufficiently intense.
Pegmatites which have been subjected to dynamic metamorphism are often not available as a source of mica, because of the distortion of the mica sheets.
Dynamic and contact metamorphism may in some cases produce rocks identical in appearance with those produced by ordinary processes of cementation and recrystallization without movement.
They may be deformed by great earth pressures, undergoing what is called dynamic metamorphism (pp.
Dynamic metamorphismis for the most part unfavorable to the development of mineral products.
The phase of metamorphismdealing with the constructive changes in rocks, due to cementation, dynamic movements, and igneous influences, is called anamorphism.
The presence of metamorphism indicating high temperatures and pressures to some extent limits the kinds of minerals which one may expect to find.
The emery of Greece and Turkey occurs as lenses or pockets in crystalline limestones, and is the result of contact metamorphism by intrusive granites.
Contact metamorphism along both the quartz-porphyry and the diorite contacts was practically lacking.
Rocks are not permanent in their condition, but at practically all times and places are undergoing some kind of metamorphism which tends to adapt them to their environment.
Hence it is that the subject of contact metamorphism is of interest to economic geologists.
Certain kinds of metamorphism are suggestive of the nature of the mineral deposits with which they are associated.
