896 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
a stream or the edge of a small lake or pond, perhaps fed by the water from a 
fumarole or siliceous spring. There appears to have been a primary vegetation of 
Rhynia Gwynne-Vaughani over the moist clayey surface. This led at places to 
a growth of peat formed in situ from the perished remains of this plant. Occasional 
sporangia from plants of Rhynia major growing in the neighbourhood fell into 
the peat as it formed. In the region represented by the main section of the bed, 
this primary covering of Rhynia Gwynne-Vaughani was arrested, never to recur, 
by the subsidence bringing water, probably charged with silica, over the surface. 
The necroses and associated wound-reactions are especially marked at this level. 
The abundance of remains of the small Crustacea and the presence of Algites Cranii 
appear to further indicate the more aquatic conditions that supervened. It seems 
probable that this provided the suitable environment for Hornea, the remains of 
which were accumulated in situ as the Hornea peat of A"l. Aster oxylon appears 
to have occurred in situ with its rhizomes penetrating this Hornea peat as drier 
conditions recurred. Then followed the first definite sandstone band, perhaps mark- 
ing a further period of subsidence and sedimentation. This is consistent with the 
presence of Hornea (mixed with the remains of Asteroxylon ) throughout Bed B 6, 
and the deposition of another definite sandstone layer above this. From this period' 
onwards the growth of Rhynia major appears to have kept pace with the sub- 
sidence, thus leading to the accumulation of a thick mass of peat. The sandy layers 
interrupting this may indicate either floodings or accumulations of blown sand — 
perhaps both. There are occasional indications of more aquatic conditions in the 
course of the accumulation of this mass of Rhynia major peat. Finally the sub- 
sidence seems to have been more active and more aquatic conditions returned, 
bringing in Hornea, which replaced Rhynia major. With increasing depth, sediments 
without plant-remains were deposited over the peat-bed. 
While this interpretation of the changes of conditions indicated by the section of 
the peat-bed studied is speculative, and it would be dangerous to press the com- 
parisons into detail, some such way of looking at the facts seems justified. It enables 
us to comprehend the mode of accumulation of the deposit in a gradually subsiding 
marsh or small lake in much the same way as such regions get filled up with peat 
at the present day. The differences from modern peats, as well as the resemblances, 
must of course be borne in mind. 
The interesting and puzzling fact of the occurrence of fragments of Nematojphyton 
in the deposit has been regarded above as supporting the view that this anomalous 
plant grew on land. How the fragments got into the peat, and what is the 
geological and botanical interest of their presence, must remain to some extent open 
questions pending further knowledge of this remarkable plant and of the Rhynie 
deposit as a whole. 
The accumulation of vascular plants in the Rhynie chert-band was clearly 
derived from a continued growth on a land-surface that was at times submerged. 
