188 PROCEEDINGS OF THE 



tain limestone and limestone shale. The coal-bearing strata in 

 South Wales have been estimated as being over 11,000 ft. thick, 

 but only about 1/lOOth part consists of coal seams proper, the 

 greater part of the thickness consisting of sandstone, clay and 

 shale. The thickness of coal seams in this country usually varies 

 between a few inches and 9 or 10 ft. Coal is formed from the 

 vegetable debris of the leaves, fronds and spores of the primeval 

 forests. In the first instance, a material was formed similar to 

 that which we find in peat-bogs, and under the influence of heat, 

 moisture and pressure from superimposed strata this peaty 

 material became compressed into comparatively thin layers, so 

 that a seam of coal only a few inches in thickness is formed from 

 the accumulation of decayed material from forests of stupendous 

 proportions. Eventually these beds became depressed so that 

 marine or freshwater sediment was deposited over them, and then, 

 again, the vegetation spread and flourished, and the same process 

 was repeated perhaps many times. The seams of coal always rest 

 upon a bed of clay known as the underclay, which represents the 

 soil on which the coal-measure plants originally grew. In many 

 cases the roots of trees are found upright in the under-clay, and 

 can be traced upwards into the superimposed coal-bed, and found 

 to be actually continuous with stems found in the upper layer or 

 coal seam. Taking the Palaeozoic Lycopods or Giant Club- 

 mosses of the coal measures. Dr. Rodman exhibited slides show- 

 ing the structure of the stem and leaves. Many species of 

 Lepidodendron and Sigillaria have been described from all parts 

 of the world, showing a remarkable uniformity in the character 

 of the vegetation. The trunks, though of slender proportions, 

 were of considerable length. Photographs showing the char- 

 acteristic leaf-scars in both genera were exhibited, taken from 

 specimens in the British Museum. The root (Stigmaria) of 

 Sigillaria was described and the relationship between stem and 

 root demonstrated. Stigmariae are always found in the under- 

 clay, and were at first described as separate fossil plants. The 

 Calamites, so called from their fancied resemblance to reeds, were 

 then described in detail, and the microscopic structure of their 

 stems shown on the screen, demonstrating their very close 

 relationship with recent Horse-tails — Equisetaceae. The very 

 interesting group of plants — Cordaites — was dwelt upon by the 

 lecturer, who pointed out that the group combined the characters 



