441 
and oilbeds may') occur alternately from the surface downward. 
Broadly speaking the filling in such a geosyncline may be said 
to begin, as a rule, with the deposition of marine sediments with 
a monotonous microfauna, later and only when these sediments 
have attained a considerable thickness, they are overlaid by deposits 
of brackish-water, of fresh-water and perhaps by terrestrian deposits 
which will or will not alternate one with the other and possibly 
also with marine deposits. 
Experience has also taught that in such marginal geosynclines 
during their long life folds may originate more or less parallel to 
the shore-line of the continent and at some distance from the shore. 
These folds may cause one or more rows of islands or a more or 
less continuous strip of land to emerge from the sea. Not seldom in 
such a case the folding process is attended with volcanic activity. 
The result may be that the portion of the geosyncline immediately 
bordering the continental shore, gets separated from the deeper ocean 
by a row of islands or a more continuous strip of land consisting of 
a system of one or more folded mountain-chains, which may even 
shut the inner portion of the geosyncline off completely, thus con- 
verting it perhaps into a freshwater lake for some time. It is evident 
that then the materials for sedimentation will be transported to the 
geosynclinal receptacle from two quarters, viz. from the continent 
and from the strip of land or mountain-range newly emerged from 
the sea, whereas prior to the folding the geosynclinal belt received 
its sediments from one side only, viz. from the pre-existing continent. 
In the case of violent volcanic action in the said strip of land, 
voleanie material will perhaps from that moment play a prominent 
part among the sediments which continue to accumulate in the 
geosyncline. 
Finally experience has also taught that the geosyncline, which 
tends to get filled up completely, now that it has become narrower and 
receives sediments from two sides, mostly undergoes itself gentle 
folding. It is well known that this folding brings about a position 
„of the strata, which is of prime importance for the working of 
oil-fields. 
The outlines of the geological history of the largest and best known 
oil-fields of the world are similar to those described just now. 
Among the numerous instances only two, the oil-fields of Pennsyl- 
~ 1) Particular stress must be laid on the word “may”, because it is possible 
that during the development of a geosyncline the conditions for the origin and 
accumulation of coal, of petroleum or of both are never quite fulfilled; in that 
case the geosyncline will remain sterile. 
