46 Proceedings of Eoyal Society of Edinburgh. [sess. 
this result has been accomplished, although perhaps it is not more 
inconceivable that a good many other cases in which organic 
structure itself of the most delicate kind has been wholly converted 
into, and reproduced in, the mineral substances of lime and silex. 
There is one process which has sometimes suggested itself to 
me, although this also is beset with many difficulties. It is con- 
ceivable that a sandy sea beach, well bored by annelid tubes, may 
have been lifted above the sea-level by some earth movement. That 
all marine deposits now converted into rock have been so lifted at 
some time or other is certain. Such movements, involving only a 
very few feet of elevation, may have been sudden, and would be 
quite sufficient. It is conceivable that the open ends of the tubes 
might be thus exposed to desiccation, and to the penetration of 
blowing sand in a dry condition, or possibly of sand carried into 
them by floods of fresh water. In either way, it is conceivable that 
the tubes may thus have been filled with grains of sand capable of 
retaining their special colouring and texture under all the subse- 
quent changes to which they, together with the surrounding matrix, 
were exposed in being hardened into rock. 
But here we come upon another fact which seems to justify all 
our difficulties — in proving that only under conditions very excep- 
tional indeed was it possible for such tubes to be preserved. In 
all parts of Scotland where this quartzite rock appears, except in 
Sutherland and Wester Ross, the annelid tubes have either been 
wholly obliterated, or else they have never existed. This last alter- 
native is highly improbable. There is good stratigraphical evidence 
that the quartzite rock which appears elsewhere to the south belongs 
to the same horizon in geological time, and must have been accumu- 
lated under similar conditions. In the island of Jura, mountains 
of 3000 feet of elevation are entirely composed of it from top to 
bottom. In Islay also, the same rock reappears, and with more of 
its characteristic whiteness than in Jura. The same strata are 
prolonged towards the north-west in the line of the Great Glen, 
or of the Caledonian Canal. A string of small islets on the 
eastern side of Lismore are composed of it, in close proximity to 
other islets of limestone against which they are faulted in a position 
of extreme unconformability. The bed which they represent passes 
straight on under the hills of Ballichulish, near which it has been 
