202 LEAVES. 



Mr. Logan, in a valuable contribution to the Geological 

 Society,* As the paper will be again cited, in treating of 

 the coal formations, we shall merely remark that the author 

 adopts the idea of Mr. Steinhauer, that the stigmaria was a 

 large, succulent water-plant, the stem, in its compressed fossil 

 state, varying from two to six: inches in diameter, and having 

 numerous processes,f which proceed vertically, horizontally, 

 and obliquely, and traverse the beds in every direction. 

 These processes have been traced to a distance of eight or 

 ten feet from the stem, and had a horizontal range of 

 twenty feet. From the number of these plants, it is con- 

 cluded that they have furnished the material for the great 

 mass of our beds of coal, and from their constant presence in 

 every seam over the whole area of South Wales, is derived 

 evidence of the most conclusive character, that the coal was 

 formed by submergence on the spot, and not by drift, as 

 heretofore supposed. J 



Eor more complete details, explanations of technical 

 terms, &c., the student is referred to the elementary works 



FIG. 141. FIG. 142. 



on botany already mentioned. As the leaves, however, con- 

 stitute a highly important feature in this department of 

 science, we shall here introduce a few remarks descriptive of 

 their characters, as they occur most commonly in fossil plants. 

 Leaves are either simple, consisting of one leaf, as in the 

 Hly; or compound, composed of several leaflets, as in the 

 rose. Compound leaves are distinguished into pinnate, 



* See Proceedings, vol. 3, No. 69, p. 275. f See fig. 101. 



Pict. Atlas, pi. xxi., xxii., xxiii. 



