338 JURASSIC. 



Below the freestone-beds at Portland there is a bed so full of 

 Serpula gordialis as almost to merit the title of Serpulite.^ Ostrea 

 duriuscida is also met with in the sands, as well as Lima rustica, 

 and Trigonia gihbosa. 



Fig. 50. — Section of the Isle of Portland. 



(C. H. Weston.) 



N. S. 



Portland Bill. 

 Weston. Southwell. Raised Beach. 



d. Purbeck Beds. b. Portland Sand. 



c. Portland Stone. a. Kimeridge Clay. 



The Portland Stone has been designated as the zone of Trigonia 

 gibbosa and Ammonites gigantciis. 



The Portland Beds are well exposed in the so-called Isle of 

 Portland, dipping gently to the south, so that, while the Kimeridge 

 Clay is exposed on the northern slopes, the stone-beds descend 

 to the sea-level at the Bill.- Purbeck Beds cover the greater part 

 of Portland to the south of the Vern Fort. (See Fig. 50.^) 



The freestone-beds are subject to great variation in thickness 

 and in quality. The accompanying general section (see Fig. 51, 

 P- 339) gives the local names applied to the beds.* 



The junction between the Portland and Purbeck strata, which 

 formerly was taken at the Dirt-bed, is now taken below the Caps 

 and on top of the Roach, as Dr. Fitton showed that by their 

 fossil contents {Cyprides, etc.), the Cap-beds belong more properly 

 to the Purbeck series.^ Nevertheless, the most important local 

 break appears in the great Dirt-bed. The Caps often form a 

 marked feature, presenting in many places a rolling or undulating, 

 and very irregular appearance, distinct from the more regular 

 beds of Portland Stone beneath ; yet in some places it is not easy 

 at a glance to determine the junction, as near Tilly Whim and 

 Durlston Head. 



Minute Gasteropods {Cerit/iiian, Delphinnla, Nerita, etc.) were discovered by 

 Mr. A. M. Wallis, in the Whit and Curf Beds." 



The stone-beds are much interrupted by fissures called "lets" or "gullies," 

 over or into which the superincumbent Purbeck strata are generally tumbled. 

 One of these gullies is represented in Fig. 51. 



The Whit Bed and the Base Bed are the chief beds of Portland Stone worked for 



1 J. F. Blake, Q. J. xxxvi. 192. 



- The Race of Portland is partly due to the agitation of the water caused by a 

 subjacent mass of Portland Stone. 



^ See also Plorizontal Sections, Sheet 20 (Geol. Survey), by H. W. Bristow. 



* Damon, Geol. Weymouth, 1884, p. 81. 



^ Buckland's Bridgewater Treatise, PI. 57 ; Buckland and De la Beche, T. G S. 

 (2), iv. 17 ; Fitton, Ibid. iv. 219 ; Webster, Ibid. ii. 37. 



* Damon, Geology of Weymouth, 1884 ; the species have been found elsewhere 

 and figured by MM. De Loriol and E. Pellat, Mem. Soc Phys. etc. Geneve, 

 xix. 192. 



