764 DK H. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 



the old land surface represented by the peat (PI. II. figs. 2 and 5). One specimen 

 (fig. 5) seems to contain a tuft of Rhynia which must have grown on the surface 

 at the time the peat bed was brought to a final close by infiltration with silica. 

 This example shows the upright stems for a length of 6 inches above the bed from 

 which they grew. The uppermost parts of the stems are embedded in almost pure 

 silica without any admixture of peaty substance, and with only a little fine, dust-like 

 matter disseminated through the silica (PL V, fig. 20). 



The whole history of the formation of the Rhynie Chert Zone, at least of that 

 portion from which our specimens were taken, can be clearly read. One can in 

 imagination see a land surface, subject at intervals to inundation, covered with a 

 dense growth of Rhynia Givynne-Vaughani. By the decay of the underground 

 parts of Rhynia and the falling down of withered stems (for this plant had no leaves) 

 a bed of peat was gradually formed varying from an inch to a foot in thickness. 

 The peat was then flooded and a layer of sand deposited on its surface. Again the 

 Rhynia covered the surface, and this process of the formation of beds of peat, with 

 the deposition of thin layers of sand, went on till a total thickness of 8 feet had 

 accumulated. 



After the formation of 8 feet of alternating peat and sand local physical con- 

 ditions must have altered, for water with silica in solution, possibly discharged 

 from fumaroles and geysers, poured over the peat bed and sealed it up. Thus the 

 whole was converted into a band of chert, the structure of the plants being 

 preserved in many cases in great perfection. It may be mentioned that the 

 presence of geysers or hot springs has been suggested by Dr Mackie as an explana- 

 tion of the occurrence of so many cherty developments in the rock series of Rhynie 

 {I.e., pp. 233-236). 



Rhynia Gwynne-Vaughani, Kidston and Lang. 



Mode of Occurrence. 



As mentioned in the introduction, the silicified peat is almost entirely formed 

 of the prostrate stems and the rhizomes of Rhynia, while in one or two fortunate 

 cases the closely crowded aerial stems were seen standing vertically (PI. II, 

 figs. 2 and 5). In these cases the ancient land surface with its vegetation is 

 recorded in a manner very rarely met with. 



The horizontal bedding of the silicified peat is distinctly seen in PI. I. fig. 1, 

 which represents a block from bed E 10 of the vertical section on page 762. In 

 the lower parts of the individual peat beds the plant-remains can be seen to become 

 more compressed than above. This is also seen in the microscopical section shown 

 in PI. Ill, fig. 9, where the stems in the upper part are round and separated by a 

 considerable amount of amorphous peaty matrix, while in the lower part they are 

 crowded and compressed. In fig. 10, from the lower part of another bed the 



