164 W. T. GORDON 
3 inches thick and the slabs placed out-of-doors for about nine months. Silica, however, 
takes a considerable time to weather, and even after all these months there was no 
appreciable difference. On experimenting with hydrofluoric acid I found that in a few 
minutes the surfaces could be etched, and after this treatment the petrifactions were 
even more clearly seen than when they had been naturally weathered, The alteration 
also was a mere surface one, and the specimens were not destroyed. So clearly did the 
specimens appear that the surface could be examined with a fairly high power—a 
4-inch objective—and even the thickenings on the walls of the tracheides could be 
made out by reflected light. It was thus possible to examine roughly the contents of 
a piece of material without making any thin sections. In this way much time was 
saved, and suitable examples of stems, petioles, and roots detected at once. 
As mentioned above, the Pettycur plants are usually preserved in calcareous material, 
and the thickenings on the cell walls are generally distinct. I have never, however, 
seen such perfect preservation as is exhibited in these silicified specimens. Locally in 
the blocks the petrifying substance had been replaced by iron pyrites, and the tissue 
could only be seen by reflected light, when it appeared like black tracery in the yellow 
matrix. On the whole the preservation is magnificent. As previously remarked, 
however, the whole matrix is very dark, and so the preparations require to be very thin 
before they become transparent. With care the sections can easily be reduced to about 
‘025 mm., the homogeneity of the material and its lack of cleavage rendering this easier 
than in the case where calcite is the petrifying medium. ‘The test used to determine 
whether the sections were thin enough, was that employed in the preparation of rock 
sections, viz. silica (quartz) grey to clear between crossed nicols. 
In a previous paper* it has been noted that the silica is present in two states, 
(a) chalcedony, and (b) quartz. The chalcedony appears to have been laid down first, and 
forms a layer round the wall of each element of the tissue. With this layer increasing 
in thickness the lumen is reduced in size. After the cell lumen is reduced to about one- 
quarter of its original size, the whole is filled in by crystalline quartz. All the silica, 
whether chalcedony or quartz, is almost perfectly transparent, and the strong contrast 
between the glassy interior of the cell and its dark walls has rendered the preparation 
of photo-micrographs much easier and more effective than in cases where calcite forms 
the matrix. In this latter case the cleavages sometimes interfere with the delicate cell 
structure, and so good photographs can hardly be obtained. 
In May of last year a more systematic study of the specimens contained in the 
fragments of the large block was begun. Several additional examples of the stem were 
obtained, and their occurrence among so many petioles might have been cited as a proof 
that stems and petioles belonged to one and the same plant. Such evidence, however, 
is of very little value and cannot be relied upon. This is especially the case when other 
genera occur in the same block. As a general rule the Pettycur blocks contain a great 
number of different genera; indeed, practically the whole flora may be represented by 
* Gorpon, Trans. Geol. Soc, Edin., vol. ix., 1909. 
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