Stations.

Total.

Depth.

Date.

lin. or

more.

more than

Olin.

Jan.

TABLE II.-RAINFALL IN 1909.

Greatest fall in
24 hours.

Days with

Number of Days on which '01in. or more was recorded.

Feb.

Mar.

April.

May.

June.

July.

Aug.

Sept.

Oct.

Nov.

Dec.

Year.

TABLE II. (CONTINUED).

Greatest fall in 24 hours.

Days with

Number of Days on which '01in. or more was recorded.

6.3 153.9 12.5 6.1 23.2 12.8 6.8 17.9 13.8 10.5 14 6 26 3 8.6 21.7 1748

..

TABLE III.-STATISTICS OF THE TEMPERATURE OF THE AIR, AND
OF THE HUMIDITY AND AMOUNT OF CLOUD, AT
WINTERBOURNE STEEPLETON MANOR,
KEPT BY MR. H. STILWELL.

AT

9

A.M.,

Temperature of the Air.

In Stevenson's Screen.

Lowest.

On Grass.

The Geology of the Furbeck Bills.

(Being the Mansel-Pleydell Prize Essay, 1909-10.)

By Dr. W. THEOPHILUS ORD.

THE geology of the district of Dorset traversed by the Purbeck Hills has been exhaustively treated by many able geologists; but it contains three problems of great interest which have not yet been satisfactorily solved. These are, in brief, (1) The mode of formation of the great Purbeck Thrust-Fault in the Chalk; (2) the origin of the Corfe Monticle on which Corfe Castle stands; and (3) the details of the vast processes of denudation by which the enormous mass of strata that within recent geological times, possibly since Oligocene days, has been removed from the southern half of the Isle of Purbeck, that is, south of the Purbeck Hills. The present paper comprises (I.) a general sketch of the geology of this district, and (II.) an account of the author's studies and researches which it is hoped may advance our knowledge of these three problems a step or two further.

PART I.

INTRODUCTORY.

The Purbeck Hills traverse the Isle of Purbeck from east to west, commencing at the chalk cliffs of Handfast Point and terminating at the chalk cliff in Worbarrow Bay. Geologically their chalk formation is continued west of Worbarrow; but since the Isle of Purbeck terminates geographically at Arishmell Gap, this paper does not consider the further prolongation of their structure west of that point. The Purbeck Hills divide the Isle of Purbeck geographically into two portions, namely a northern half which extends to the River Frome, and consists of Tertiary strata, chiefly Bagshot sands and clays, and a southern portion which forms a syncline of Wealden Clay merging southwards into the northern slope of an anticline of Upper, Middle, and Lower Purbeck beds ending at a coastline of high cliffs of Portland stone and sand resting on Kimmeridge clay, which forms the axis of the anticline. The line of Purbeck Hills thus dividing the Isle of Purbeck is formed of an elevated monoclinal ridge of highly inclined, often vertical, chalk, flanked on the north by Tertiary beds, and on the south by a narrow strip of Greensand, the beds of which are conformable with the almost vertical chalk, and hence are exposed at the surface on edge. These comprise Punfield beds, Atherfield clay, and Upper and Lower Greensand, which, however, can seldom be distinguished geologically. The Gault is not here represented as distinct from the Upper Greensand.* The three diagrams on Plate I. give an illustration of the relationship of the strata in three transverse sections-I. near the eastern extremity of the hills; II., about their centre; and III., near their western termination.

* Strahan, Memoirs Geol. Survey, Isle of Purbeck, p. 143.