This article attemps to coordinate the various aspects of the development of a geosyncline, the general definition of which is based, in this article, mainly on orogenic rather than on sedimentological features. Mio- and Eu-geosynclines are distinguished.

Geosynclinal sedimentary environments may be grouped as follows: A. The mio-geosynclinal environment comprising: a) the carbonate platform association, b) the clastic shelf association, c) the mixed shelf association. B. The slope environment: a prominent, persistently highly mobile, structural zone forming, as a rule, marked submarine scarps; the mobility prevents an undisturbed accumulation of sediments and both gravity gliding and turbidity currents occur with great frequency here. C. The eu-geosynclinal environment comprising two main assemblages: a) the perennial group
consisting mainly of shales, red clays, oozes, limestones and cherts; b) the catastrophic group which is generally accumulated by gravity glides and turbidity currents. Most of the Flysch and probably also greywacke associations may be included here. However, the distribution and thickness pattem of sediments in a eugeosyncline are, in our opinion, only a secondary diagnostic value, the magmatic content and effects directly related to the subcrustal development of the mega-unit being fundamentally much more characteristic.

The most representative feature of a eu-geosynclinal belt is the presence of considerably differentiated basic and ultrabasic igneous rocks, the socalled ophiolites. These occur mainly along the hinge zones and may, like the sediments, glide into the eu-geosyncline in the form of olisthostromes. 'Thus the complicated structures that often accompany such olistbostromes must be considered partly to have been caused by this gliding rather than by regional deformation.

Most of the world’s oil reserves occur in shelf sediments and it would seem, on circumstantial evidence, that eu-geosynclines have but few prospects. However, exploration is only rarely carried into the highly disturbed belts of the en-geosynclines and there exist a number of specific cases where more investigation is needed,