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Geosynclines are surfaces of regional extent that subside considerably during formation of their included surficial rocks; they have limits in time and space. Early Paleozoic North America had a rather stable center (hedreocraton) margined by deeper sinking belts (miogeosynclines) that initially received carbonate rocks, and quartz sands from the interior; neither area had appreciable volcanism. The continental borders have distinctive volcanic flows and fragmentals, which with associated sediments show deep subsidence (eugeosynclines) and development of associated tectonic welts. Lands raised in the eugeosynclinal belts yielded sediments to the adjoining miogeosynclines; with deformation of the latter, terrigenous detritus spread into subsiding areas (exogeosynclines) in the margin of the hedreocraton. The craton periodically gained basin or trough-shaped depressions isolated from highland source areas (autogeosynclines) or receiving debris from associated intracratonal elevations (zeugogeosynclines).

The stratigraphy of the rocks in Paleozoic eugeosynclines shows that they were formed in regions much like modern island arcs. Some of the ancient eugeosynclines are continuous in trend with modern volcanic archipelagoes, which lie on the continental side of the ocean basins. The eugeosynclines became consolidated by orogeny and by intrusion of plutonic rocks; ultrabasic intrusions are restricted to them, and great batholiths are predominant within their areas. Deeply-subsiding, relatively non-volcanic troughs (epieugeosynclines) developed later, and were succeeded by fault-bounded depressions (taphrogeosynclines). The belts became rather stable additions to the craton. They finally developed coastal plains passing marginally into coastal geosynclines (paraliageosynclines).

It is axiomatic that nearly all kinds of surficial rocks are thickest in geosynclines; geosynclinal facies are characterized by thickness rather than kind. The better sorted sediments virtually are restricted to the cratons and adjoining geosynclines, and volcanic detritus is prevalently in eugeosynclines and the proximal parts of miogeo-synclines. Because of the mobility of lands in eugeosynclinal belts, poorly sorted detritus, such as conglomerate and graywacke, is most abundant, but carbonate and siliceous rocks prevailed over large areas for long times.

Plutonic rocks are extensive in the pre-Paleozoic and volcanic rocks are widespread. Thus, by analogy, each part of the continent has in some past time been eugeosynclinal. Eugeosynclines and modern island arcs are thought to be in areas of thin sial. Sialic nuclei grew through orogenic consolidation of eugeosynclinal belts, formed at the expense of an originally universal simatic crust, of which the ocean basins, the low cratons, are relics. The nuclei coalesced into a single continental mass by earliest Paleozoic, when thin-sialic eugeosynclinal belts extended to the present continental margin. Stratigraphic evidence contradicts hypotheses that assume an original floating sialic block on surrounding and subjacent sima; the continent has developed through dynamic processes.

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