56 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



large lake. Circumstances have occasionally rendered possible 

 the preservation of old land-surfaces with the stumps of trees 

 still in their position of growth. One of the best examples of 

 this in Britain are the so-called dirt-beds or black bands of 

 Portland and the Dorset Coast. On the cliffs immediately east 

 of Lul worth Cove, the surface of a ledge of Purbeck limestone 

 which juts out near the top of the cliffs, is seen to have the 

 form here and there of rounded projecting bosses or 'Burrs' 

 several feet in diameter. In the centre of each boss there is 

 either an empty depression, or the remnants of a silicified stem 

 of a coniferous tree. Blocks of limestone 3 to 5 feet long and 

 of about equal thickness may be found lying on the rocky 

 ledge presenting the appearance of massive sarcophagi in 

 which the central trough still contains the silicified remains 

 of an entombed tree. The calcareous sediment no doubt 

 oozed up to envelope the thick stem as it sank into the soft 

 mud. An examination of the rock just below the bed bearing 

 these curious circular elevations reveals the existence of a 

 comparatively narrow band of softer material, which has been 

 worn away by denuding agents more rapidly than the over- 

 lying limestone. This band consists of partially rounded or 

 subangular stones associated with carbonaceous material, and 

 probably marks the site of an old surface-soil. This old soil is 

 well shown in the cliffs and quarries of Portland, and similar 

 dirt-beds occur at various horizons in the Lower and Middle 

 Purbeck Series \ In this case, then, we have intercalated in a 

 series of limestone beds containing marine and freshwater 

 shells two or three plant beds containing numerous and fre- 

 quently large specimens of cycadean and coniferous stems, 

 lying horizontally or standing in their original position of 

 growth. These are vestiges of an ancient forest which spread 

 over a considerable extent of country towards the close of the 

 Jurassic period. The trunks of cycads, long familiar in the 

 Isle of Portland as fossil crows' nests, have usually the form of 

 round depressed stems with the central portion somewhat hol- 

 lowed out. It was supposed by the quarrymen that they were 

 petrified birds' nests which had been built in the forks of the 

 1 Woodward, H. B. (95), Figs. 124 and 133 from photographs by Mr Strahan. 



