THE HIGHLAND BORDER ROCKS OF THE ABERFOYLE DISTRICT. 
193 
that the rock is organic in character, but the organisms have been considerably 
altered in fossilisation, and minute structural details are rarely recognisable. 
“ The sections are noted in the order in which they are marked. 
“1. About one-third of this section is a coarsely crystalline rock, mostly without 
organisms ; the larger part consists of small fragments of organisms, some of 
rounded or oval forms, with walls either entire or perforated by small canals ; the 
interiors are either of crystallised rock or of fragmental organic debris. These 
bodies may be Foraminiferal in origin. Intermingled with them are numerous 
elongated fragments, dark in appearance, probably hollow originally, which seem to 
be organic, but I cannot say to what organisms they belong. (Plate II, fig. 11.) 
“ 2. The rock section is like the preceding — in large part of coarse-grained 
crystallised limestone, in which are embedded fragments, irregular in outline, of 
fossils of very minute calcite grains. Some of these are very probably calcareous 
Algae. The marked difference between the coarsely crystalline limestone matrix and 
the extremely minute grains composing the fragments of calcareous Algae can be 
well seen in this slide. (Plate II, fig. 13.) 
“ 3. Here again the difference between the very fine character of the grains of 
the portions which were probably calcareous Algae and that of the calcite matrix can 
be clearly seen. 
“ 4. This rock section is of comparatively similar grains of limestone, and no 
fossil fragments appear therein. 
“ 5. A large part of the rock consists of rounded organisms with walls perforate, 
also of elongate bodies, and these fossils are of limestone in extremely fine grains. 
The rounded bodies are apparently Foraminifera.” (Plate II, fig. 12.) 
VII. The Age of the Highland Border Rocks. 
The striking resemblance of the members of this marginal strip of rocks to the 
Arenig succession in the South of Scotland has already been remarked upon. This 
similarity is evident not only in the black shales and cherts, but also in the 
associated igneous rocks, both lava-form and intrusive, and in the highly meta- 
morphosed bands.* This lithological resemblance led Messrs Peach and Horne to 
correlate the rocks of the two regions, and consequently the Highland Border Rocks 
are marked as doubtfully Lower Silurian on the Geological Survey Maps. 
The assemblage of fossils obtained from the cherts is strongly suggestive of an 
Upper Cambrian or Lower Ordovician horizon. The brachiopods and phyllocarid 
crustaceans are represented by genera and species which are characteristic of Upper 
Cambrian and Lower Ordovician rocks elsewhere. A few of the brachiopods have 
been noted hitherto only from the Middle Cambrian. The graptolites, on the other 
hand, point to an Orfipvician horizon. Taking the evidence from the fossils as a 
* The Silurian Rocks of Britain , vol. i, Scotland (Mem. Geol. Survey), p. 46, 1899 ; Sir A. Geikie, Ancient 
Volcanoes of Great Britain, vol. i, p. 201, 1897. 
