^ Mr. Webster on the Purheck and Portland Beds. 



nous clasSj which are from 1 to 2 feet in diameter. The woody part is siliceous^, 

 and the longitudinal vessels are filled by^ and surrounded with^ radiated quartz : 

 numerous veins of chalcedony and quartz also pass through these stems^ but 

 always following the direction of the concentric and radial structure. In the 

 cavities, and particularly on the outside, there is a small quantity of carbonate 

 of lime ; but this is only superficial, and has been a deposit subsequent to the 

 silex. I saw one of these trunks standing erect, and the workmen informed 

 me they were frequently found in this position : its lower part was thickest, 

 and being divided, it gave the idea of the commencement of roots : its upper 

 part penetrated through the soft burr and terminated in the aish. The trunks 

 are always broken off short, seldom exceeding 3 feet in length. This is the 

 fossil wood which is so often brought from the Isle of Portland ; and upon the 

 most careful inquiry and examination, I could not discover that it was found 

 in the oolite itself, nor in any other part of the series. In this earthy bed are 

 also many stones evidently water-worn, which I afterwards ascertained to be- 

 long to the lower part of the Portland series ; one was a dark grey splintery 

 limestone, and another a brown stone slightly oolitic. 



The bed below this is called the Top Cap, and varies considerably in its 

 structure. Some parts of it are entirely compact ; in other places it contains 

 compact parts imbedded in a softer rock; and in others again it is slightly cel- 

 lular ; but, as far as 1 could discover, contains no fossils. 



The next bed is called the School Cap, and is of a very remarkable struc- 

 ture ; it consists of a compact limestone extremely cellular, the cavities being- 

 almost filled with groups of rhomboidal crystals of carbonate of lime. 



Under the School Cap is a layer 6 inches thick of flint, or chert, containing 

 shells, and often having also oolitic grains imbedded. 



The bed below this is the first which is worked for freestone, and is called 

 Roach. Its thickness is variable; in the quarry which I measured, it is 15 

 feet thick, and without subdivision. This is the most valuable bed, and blocks 

 of vast size are raised from it for the London market. It is entirely oolitic, 

 and generally of a very uniform quality. It contains marine shells, as oysters, 

 various bivalves, and likewise the Hallirhoe of Lamouroux, but the organic 

 bodies are not numerous. 



The next bed, called the Ruhbly Bed, is remarkable for the quantity of casts 

 of shells which it contains. This renders it unfit for building, except for fill- 

 ing-in thick walls and foundations. It is also oolitic; and, except that the shells 

 occasion cavities, is nearly as firm as the Roach. 



Below the Rubbly bed is another of excellent stone, but sometimes rather 

 harder than the Roach. This is about 6 feet thick, and is also worked when 



