SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 893 
(c) Rhyviia Gwynne-Vaughani has only been met with at, and slightly above, 
the base of Bed A'T, i.e. at the extreme base of the deposit. Asteroxylon , except for 
an isolated tracheide in Bed F 116, has not been met with above Bed B 6. Hornea 
occurs in the lower beds up to Bed B 6, and reappears in the uppermost Beds M and 
N. Rhynia major is only an occasional and relatively unimportant component of 
the beds up to B 6. Above this from D to L it forms the whole deposit, being then 
gradually replaced by Hornea in the uppermost beds. 
( d ) The change in composition of the plant-remains above the Rhynia Gwynne- 
Vaughani zone at the base takes place without any intervening sandy layer. The 
succeeding peat may be composed of Hornea with some Rhynia major, but this in 
other blocks is represented by loosely packed Hornea and Asteroxylon, or by Aster- 
oxylon only. 
(e) The peat-beds are separated and interrupted by sandy layers of two sorts : 
(l) There is a very definite bed of sandstone separating the characteristic remains of 
A"1 and A"2 from the different type of peaty deposit in B 6. Another definite sand- 
stone layer occurred at C, and was followed by the succession of beds with pure 
Rhynia major. (2) The separation of the other peat-beds is due to an accumulation 
of sand mixed with abundant plant-remains, the decay of which has been more or 
less complete. The passage of the plant-remains into thin carbonaceous partings 
can be followed at places. The less marked interruption of the peat-beds is due to 
similar irregular accumulations of sand and, associated with this, more complete decay 
of the vegetable tissues. Occasional isolated grains of sand in the peat itself indicate 
the other extreme of the association of sand with the peaty mass. 
(/) There are indications in the different nature of the preservation of the stems 
and from associated algal remains of more aquatic conditions at certain levels. This 
can be regarded as probable for the region above Rhynia Gwynne-Vaughani in A'T, 
and for Beds D and F 116. Possibly the presence of Hornea is another indication 
of moister conditions : in connection with this the reappearance of this plant in 
Beds M and N is to be noted. 
(g) The loose packing and excellent preservation of the plant-remains in the 
basal beds is a remarkable feature of the deposit. There is no regular increase in 
decay and compression on passing from the upper peat-beds downwards. 
Certainty as to the exact conditions of accumulation of this ancient deposit is 
probably unattainable. Our knowledge of the details of accumulation of various 
deposits at the present day, which might afford further help, is only sufficient to 
serve for the institution of general comparisons. A consideration of the facts 
summarised above with some others appears, however, to justify a further interpre- 
tation of the story to which the following remarks are a contribution. 
The more detailed examination of the peat-like bands themselves has supported 
the interpretation of their nature which is expressed in the term “ silicified peat” used 
throughout these papers. In some cases it is clear that the plants grew in the 
