PORTLAND DIRT-BED. 



a layer about one foot in thickness, composed of dark brown friable- 

 loam, containing a large proportion of earthy lignite, and, like the 

 recent soil of the island, many water-worn stones and pebbles. 

 It seems to have been a bed of vegetable mould, which at a remote 

 geological epoch supported an abundant and luxuriant vegetation,, 

 for we find in it and upon it innumerable trunks and branches of 

 cone-bearing trees and cycadeous * plants. Above this bed are 

 found layers of finely-laminated cream-coloured limestones, the 

 total thickness of which is about ten feet, and upon which is 

 deposited the modern vegetable soil ; but this latter at present,, 

 instead of supporting cycadeous plants and pine forests, barely 

 maintains a scanty vegetation. 



The most remarkable circumstance attending this dirt-bed, as it 

 is called, is the position of the trees and plants found on it. They 

 are still erect, as though they had been suddenly petrified while 

 growing in their native forests, with their roots in the vegetable 

 soil and their trunks extending into the limestone above it. 



Immediately below it is a thick stratum of fresh-water lime- 

 stone, of little value for building ; and below this again is the 

 stratum of the celebrated Portland stone so extensively used for 

 that purpose. The consequence is that the dirt-bed and its- 

 interesting materials, little regarded by quarrymen, are cast 

 away and scattered about as mere rubbish, in order to get at 

 the layer of building-stone which lies below them. " On one of my 

 visits to the island (in the summer of 1832)," says Dr. Mantell, 

 " the surface of a large area of the dirt- bed was cleared, prepa- 

 ratory to its removal, and the appearance presented was most 

 striking. The floor of the quarry was literally strewn with fossil 

 wood, and before me was a petrified forest, the trees and plants,, 

 like the inhabitants of the city in Arabian story, being converted, 

 into stone, yet remaining in the places which they occupied when 

 alive ! Some of the trunks were surrounded by a conical mound 

 of calcareous earth, which had evidently, when in a state of mud, 

 accumulated round the stems and roots. The upright trunks, wore 

 generally a few feet apart, and but three or four feet high ; their 

 summits were broken and splintered, as if they had been snapped 

 or wrenched off by a hurricane, at a short distance from the ground. 

 Some were two feet in diameter, and the united fragments of one 

 of the prostrate trunks indicated a total length of from thirty ta 

 forty feet ; in many specimens portions of the branches remained 

 attached to the stems. In the dirt-bed, there were numerous 

 trunks lying prostrate, and fragments of branches. 



" The external surface of all the trees I examined was weather- 



* Such as palms and ferns. 



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