56 



formed of this material are arranged ; and (3) the agents used in 

 modifying the rocks, namely — sea, atmosphere, and rain. A glance 

 at a geological map of the South Coast shows that along Dorset an 

 encircling band of hard Jurassic rocks surrounds and protects the 

 softer younger rocks, curving to the south-west. Hence stand out 

 the promontories of Portland, the Isle of Purbeck, etc. ; whilst 

 where this has been destroyed the sea has eaten the coast into bays, 

 as at Weymouth, Swanage, Lulworth, etc. Similarly, the chalk 

 surrounded the younger Tertiary beds of the Hampshire basin, 

 until the sea broke through into Bournemouth Bay and partially 

 washed them out. The harder rocks stand out as headlands at 

 Portland, St. Aldhelm's Head. Peveril Point, Ballard Downs, the 

 Needles, etc. Whilst the sea is chiefly responsible for carving out 

 coast scenery, land streams assist by forming coastal valleys and 

 washing out debris. Ice also helps in winter by forming in fissures 

 and then splitting off masses of rock from high up on the cliffs ; 

 whilst in limestone and chalk strata the lime is dissolved by rain 

 water and fissures formed. Similarly, joint planes are widened, and 

 so all contribute to the action of the waves in forming our shore 

 scenery by breaking down the cliffs, carving out bays, leaving 

 headlands, and so on. 



Coast erosion is largely determined by the folds of the strata, 

 the direction in which the beds tend to split up, and the joints into 

 which rocky cliffs divide. The position of these, relative to the 

 direction of wave force, is very important. Horizontal lines of 

 stratification with large joints in chalk and limestone tend to form 

 stacks and pinnacles, as at Old Harry Rocks, the Needles, and 

 Duncansby Head. Earth movements influence coast scenery by 

 producing bedding-planes in the rocks, along which they split very 

 easily. Where the strata have been much compressed and con- 

 torted by earth movements, great irregularity of rock scenery 

 results. This may be seen at Durdle Door and Stair Hole, near 

 Lulworth, as well as in the rocks guarding the entrance to 

 Lulworth Cove. 



The opposite of these effects is seen in the coast lines of softer 

 material, when we get wide, low harbours, and beaches protected 

 by sand and mud banks. These are seen in Poole Harbour and 

 Bournemouth Bays. 



At the close of this demonstration a hearty vote of thanks was 

 given to Dr. Ord for conducting the classes, which had proved of 

 great interest and were well attended by many who heard Mr. 

 Spicer's lectures, as well as members of the Natural Science Society. 

 The classes were of an informal nature ; questions were freely 

 asked, and some interesting discussions took place on points of 

 difficulty. This concluded the work of the Geological Section for 

 the W T inter Session. 



. The first geological excursion of the Summer 



xcurston o Session, 191 1, was arranged in conjunction with 

 p y ' the Botanical Section on Wednesday, May 3rd. 



Owing to a gale, the steamer did not go, and the few members — 



