Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/378

 A L P s what it has been. Formerly most geologists supposed upon a fundamental crystalline mass of much greater that all schists (using the word in its strict sense of antiquity. This appears to be separable into two groups, foliated crystalline rocks), together with saccha- generally, though not always, rather sharply dis- e the Meta ' roidal marbles and gneisses, resulted from the tinguished. The lower one consists mainly of ^ °Jee morp sm. apei.a^jon 0f sediments by the combined action granitoid rocks and gneisses, the majority, if ro^ of heat, pressure, and water; some even going so far as not the whole, being igneous rocks modified to maintain that certain igneous rocks represented the by subsequent pressure. Some are later in date than extremest stage of this process. In their opinion a mud, if others; for instance, the protogine of the Mont Blanc of the right chemical composition, might become in the range, long supposed to be the oldest rock, is clearly first stage a schist, in the next a gneiss, in the last a intrusive in the adjacent gneisses. The upper group ia granite. This kind of change, it was believed, had mainly composed of mica schists, commonly calcareous, occurred during every geological period, so that, in the passing on the one hand into marbles, on the other to one or the other of these, schists and gneisses were identified quartz schists; being thus metamorphosed sediments. With without hesitation. Hence Mr Ball not infrequently these are associated, sometimes abundantly, hornblendic mentions various Palaeozoic schists. But, whatever may be and chloritic schists, in great part at least modified, the case elsewhere, we may venture to affirm that in the igneous rocks. Bocks of similar character occur also, Alps all the rocks thus modified are much more ancient though less frequently, in the other group; and normal than any to which a date can be assigned. Though in granites, gabbros, and serpentines in both, the age of most districts we can only prove them pre-Carboniferous, which has not yet been certainly determined. These rocks, the foundation-stones of the Alps, are in the north-eastern region they are pre-Silurian, and in all it is highly probable that they are older than the followed, but so far as is known only in the north-eastern Pakeozoic era, as usually understood. Every effort to region, by sedimentary deposits of Silurian and Devonian strengthen the old hypothesis, and not in the Alps only, age. Bepresentatives of the Carboniferous system are has signally failed. For instance, in 1888 two large more widely distributed, though generally in isolated stems of a fossil plant were asserted to have been found strips, folded in among the crystalline masses. These are in a gneiss at Guttannen in Canton Berne, thus proving commonly dark in colour, and vary from good slates to it to be Carboniferous in age; in 1900 this supposed coarse conglomerates or breccias, which often are full of organism was admitted to be a mere lusus natures. In pieces of the older schists and gneisses. They occasionally 1888, also, garnets and staurolites were said to occur in contain plant remains or a little anthracite, but marine a rock in which belemnites and crinoids could still be fossils occur in the Gailthal and elsewhere in the Eastern recognized, thus establishing the Jurassic age of a great Alps. The Permian system also is generally represented group of schists. In 1890 it was demonstrated that this sporadically, as a rule by grits and conglomerates, indifar-reaching hypothesis had no better foundation than an cating that high ground existed in the neighbourhood;. erroneous identification of the two minerals, those actually but during this period the region of the South-eastern Alps was the scene of volcanic activity, which produced present having no real bearing on the question. Of the advances, mentioned above, none is more the great masses of “ porphyry ” around Botzen and Preimportant than the recognition of the effects due to severe dazzo, which can be traced westwards beyond the lowerpressure. That it had produced cleavage in end of Lago Maggiore. Outbreaks of this age can be Effects of ordinary sediments was long since generally detected in other localities, as on the Windgelle and to the of Vernayaz, on the right bank of the Bhone. In o^Alplae ^mittsd; that in some cases it had generated east rocks. minute scales of mica in vast numbers and South-eastern Tyrol the volcanic discharges continued into converted a slate into a “ microscopic ” schist the Triassic period, and were renewed, in the opinion of was more than conjectured; but it is now ascertained that some geologists, at a much later date. In this district crushing due to the great earth movements which have the Trias is represented by sedimentary strata, consisting formed mountain chains has produced cleavages even in largely of pure dolomitic limestones, some of which have crystalline igneous rocks, and led to the development of been thought to be coral reefs. Be this as it may, thevarious minerals, especially in connexion with the new existing mountains can hardly represent ancient atolls, as surfaces, so that many gneisses and some mica schists some have supposed. At this period, also, the surface' are only granite modified by subsequent pressure. Other contours of the Alpine region must have been very irregular, gneisses, however, and certain cognate schists, especially for in other parts, notably in the Pennine, Central, and those with a conspicuous mineral banding, are the result Dauphine Alps, the Trias is either absent or is represented of fluxional movements in the magma at the time of by friable limestones and even gypsum, often not moreconsolidation; while a third group of schists, and possibly than a few feet in thickness. some gneisses, may be explained by the old hypothesis. In regard to the rest of the Mesozoic and the Kainozoic One difficulty doubtless still exists : in certain instances it systems, little need be added to the account given in the is almost impossible to determine whether a very fine ninth edition of this work, except in regard to a perplexing grained schist represents a stage in the last-named process or deposit called the Flysch. It is a thick mass of mudstones, results from the pulverization of a crystalline rock. This more or less gritty and generally unfossiliferous, which, it is uncertainty, however, does not vitiate the general state- now ascertained, represents identity of physical conditions ment that a group of true schists can be distinguished rather than exact contemporaneity of deposit. In the from one composed of sediments in which changes only northern part of the Eastern Alps it begins before the of microscopic magnitude have occurred, neither does it close of the Cretaceous period ; in the southern its greatest prove that the two may be present in the same geological extent is from the top of the Middle Eocene to that of the formation ; it only shows that in certain cases nature has so Middle Oligocene. In the Western Alps it ranges, in the damaged her own inscription that it is no longer legible. Swiss portion, from the bottom of the Middle Eocene to Thus, the following facts in regard to the nature and the top of the Lower Oligocene, while in the south age of the Alpine rocks would now be generally admitted it does not begin till the Upper Eocene. It conby those who have studied them in the field as well as tains locally beds of breccia and even large isolated under the microscope. Those rocks (mainly sedimentary) boulders, the volume of which is sometimes, as in the which can be assigned to a definite geological age rest Habkernthal, several cubic yards, representing granites 332