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Ch. 1: Introduction

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INTRODUCTION
19
in the Dalradian sediments of Loch Arne in Scotland and the archsean nucleus of Saxony.
The justification of assigning the Manhattan Island schists to metamorphosed Hudson River slates is doubted.
Again, Albert Heim has indicated the extreme physical changes brought about in sediments by mountain uplifts and compression: " Enormous zones, for instance, in the interior of the Finsteraa massif, that were formerly held to be true crystalline schists, prove to be originally plastic rocks of the carboniferous period that have been squeezed into schists and pervaded by secondary, mica. Conglomeratic rocks of the Verrucano group and clay-slates nipped into the central massif have become crystalline schistose, and even gneissose. They can scarcely be distinguished, in the field and in the hand specimens, from crushed gneisses pervaded by sericites. Granites can be proved locally, and perhaps also regionally, to have been compressed into gneisses. Gneisses, having a different position relatively to the pressure, have locally be­come granitoid. Massive eruptive felsite-porphyries have be­come felsite-schists. Mica-schists have been dragged out, their quartz grains ground down, and the whole converted into a rock that one would be inclined to describe as a sandy clay-slate. Even Liassic slates, with fossils, have been con­verted into garnetiferous mica-schists, staurolite-schists, etc. The boundary between the old crystalline schists and real sedi­ments in the Alps has, by such processes of dynamic meta-morphism, been obliterated."
Metamorphism must not be associated exclusively, or even predominantly, with the alteration of soft or unconsolidated beds of sediments. Its effects are quite as evident in rocks which have become completely solidified and which have al­ready advanced to a crystallized or semi-crystallized stage, as in slates, limestones, and schists, sandstones, even granites, diorites, and dolerites.
A. C. Spencer has described the metamorphism of a con­glomerate in the Encampment District of Wyoming, which
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