Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/186

 Such, then, is a brief account of the strata and fauna of the Lower Eocene deposits as seen in the Portsmouth sections. It remains to determine the position which these beds may be supposed to occupy in relation to the cliff-sections at Alum Bay and Whitecliff. The evidence of the fauna, including as it does many species respectively of a high and low level in the Eocene strata, is too contradictory to be of much value. The stratigraphical evidence which it is my good fortune to be able to lay before the Society, however, is perfectly conclusive.

I propose to show this in two ways : — First, by comparing the ascertained rise per hundred feet of the beds described with their distance from the known outcrop of higher and lower formations ; and, secondly, by the more direct evidence of borings and well-sections.

Evidence of Thickness of Beds by Horizontal Measurement.

I have shown that the rise of the strata along the line of the section (fig. 1), from south to north, equals a rise of about 1 foot in every 22 feet. Now the distance from the outcrop of the highest beds exposed on the works to the known outcrop of the underlying Red Clay is about 6000 feet, which gives a thickness of about 275 feet to the intermediate beds, or within 25 feet of the thickness of the London Clay and Bognor series of Whitecliff. Then, again, the distance from the line of strike of the higher beds on the works to the nearest known outcrop of the Bracklesham beds is about from 6500 to 7000 feet, giving a thickness to the intermediate strata of from 295 to 315 feet. And this thickness agrees very nearly with that of the Lower-Bagshot strata at Whitecliff. Supposing these measurements to be correct, the strata above described must represent nearly the upper portion of the London Clay of the Whitecliff and Alum-Bay sections.

Evidence by Borings and Well-sections.

The thickness of the strata exposed on the works in open cuttings I have shown to amount to about 127 feet. A boring has been recently sunk from almost the lowest of these beds to a further depth of about 115 feet (fig. 3, b). The details of this boring are as follows : —

Details of boring in Extension Works, ft. in.

Sandy clay 10 0

„ with more clay 5 0

Stiff clay 5

Very stiff clay ( = " Clay with Ostrea " of the Section) 18 0

Claystone 2

Very stiff clay 29 0

Layer of Pyrites and a small shell (Turritella) 1

Stiff clay 12 8

Claystone 2

Clay 2 0

Claystone 2

Very stiff clay, 10 10

Clay full of rounded black pebbles 9

Hard sand 22 3

Blue clay 39 0

Total 155 1