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SECTIONAL TRANSACTIONS.—C. , 377 
essential feature of the Jurassic and early Cretaceous history of the region. It is 
displayed by the maps showing the conditions in Selbornian and again in Turonian 
times, but the loci of maximum sedimentation are to be found further and further 
east with the progress of time, strongly suggesting a systematic migration away from 
the source of detrital material. 
In the second type, the downwarped areas are to be regarded as compression 
subsidences foreshadowing the London and Hampshire basins. This type is first 
evidenced in the Cenomanian epoch, but-the compressional forces that produced it 
must have died away in Turonian times when a reversion to the simpler conditions 
typical of the Jurassic era took place. They were regathered more intensely in the 
Senonian epoch when the tectonics assumed a definitely Tertiary aspect. Later, 
as is well known, the two compression subsidences whose first beginnings are traced 
here, were further depressed by heavy loads of sediment. 
Intimately connected with the eastward migration of the centre of maximum 
sedimentation and depression is the progressive tilting of Britain that has gone on 
ever since Cretaceous times. It results from the progressive loading of the eastern 
regions and is evidenced by the westward convergence of the principal Cretaceous 
and Tertiary datum planes, and by their successive intersections. 
Finally, the validity of the conclusions drawn from the isopachyte data is estab- 
lished beyond doubt, by the ability of the method to demonstrate the existence of a 
series of infra-Cretaceous minor folds, whose reality is abundantly confirmed, although 
it could hardly have been deduced, by the paleontological records. 
Dr. E. O. Uxricu. 
The writer has enjoyed exceptional opportunities and devoted much time in 
the past 35 years to the study of stratigraphic phenomena indicating shifting of 
areas of marine deposition because of earth movements. In the case of structural 
domes the movements usually consisted of alternating elevation and subsidence 
and of differential tilting of their surfaces ; in the case of geosynclines they originated 
and subsequently, with minor modifications of trend and complexity, they developed 
and largely maintained more or less definitely located persistent longitudinal major 
troughs and embayments, the bottoms of which were at times submerged, at other 
times elevated completely or partly above sea level ; and in the case of the wide and 
nearly flat interior areas by differential tilting of the general surface of the continent. 
As arule, the physical evidence on which shifting of epicontinental seas is inferred 
consists (1) of the pinching-out of a formation so that normally overlying and under- 
lying stratigraphic units come into contact ; (2) the reduction in thickness is mainly 
or solely by loss from below so that the upper beds extend farthest—an impossible 
condition under the conception of loss by erosion prior to the deposition of the 
overlying formation. The organic evidence indicates alternating occupancy of the 
epicontinental troughs and shallow basins by waters and faunas of diverse sources 
and direction of invasion. Namely, the fossil remains in the concerned formations 
indicate more or less positively that the often directly superposed faunas originated 
and developed in, and at times of submergence invaded the submerged areas, in some 
instances from the north, in others from the east, and in the remaining cases from 
either the south or the west. Consequently, the organic remains in two or more 
widely separated formations in a continuous outcrop of, say, Ordovician deposits, are 
so closely similar in generic and even in specific relations, that no doubt can obtain 
as to their common origin and the sameness of the oceanic realm from which they 
invaded the continental basins. However, between these recurring invasions of, say, 
a northern or an Atlantic fauna, the section commonly displays other faunal zones 
that may either simulate each other or differ radially, but in neither case be at all like 
that, or those, of the first set. These intercalated faunas, therefore, must have 
originated in and invaded from such other oceanic sources as the Gulf of Mexico or 
the Pacific side of the continent. 
Many such instances of shifting of seas because of earth movements in the 
Appalachian Valley and on the flanks of the Cincinnati and Nashville domes and on 
the slopes of the Adirondacks and Ozark uplifts were brought out and discussed in 
the writer’s Revision of the Paleozoic System (1911), and others observed in 
Wisconsin were published in subsequent papers. On the present occasion most of 
