26 Mr. Witham on the Vegetation of the First Period of the World , 



with it. It must be observed that in almost every instance 

 they are surrounded by a coating of very fine coal, of about 

 half or three-quarters of an inch thick, having a polished sur- 

 face, with veiy little attachment to the surrounding matter. 

 This, I doubt not, is the cause of the fall ; the fossil dropping 

 out sometimes as much as three feet in length, leaving a 

 hole in the roof almost perfectly circular. Often it falls in 

 these large pieces ; but sometimes the nature of the shale of 

 which its substance is composed, causes it to fall in portions 

 of different thickness. It is to these falling pieces that the 

 miner's expressive term Kettle Bottoms applies. 



These fossil plants run from two to eight feet in circum- 

 ference. The occurrence of numerous impressions which you 

 may observe in the specimens of parts of different plants in 

 the shale, forming the substance of these fossils, is to me, I must 

 confess, very difficult of explanation. Some years ago a friend 

 of mine found a kettle bottom at old Kenton colliery, eighteen 

 inches in diameter, coated with fine coal, the substance of 

 which was entirely mineral carbon or charcoal, with a mixture 

 of earthy matter and pyrites. A portion of this specimen is 

 in the collection of the Geological Society. It is much to be 

 regretted that hitherto none of these interesting fossils have 

 been followed into the strata. We do not know how far they 

 extend, or to what height they are standing. 



Again, in the coal districts of Scotland, amongst the troubles 

 which affect the roofs of coal, there is one, of a very singular 

 form, known by the name oi Pot Bottom or Cauldro7i Bot- 

 tom, and are from the size of one foot to five feet in diameter. 

 One of these is represented in the annexed diagram. 



A. Roof of coal, argil with sand. 



B. Pot or cauldron bottom. 



C. Bituminous coal. 



D. Pavement of coal. Fire clay. 



In 



