PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 415 
liquid phases was preserved over a long period, to effect a gradual 
dolomitization. A concentration of magnesium salts in such a 
case is hardly possible, as evaporation could not have taken place; 
the result must be attributed entirely to long-continued favorable 
physical conditions. This is the explanation that first suggested 
itself; but it is hardly tenable. If such a process does actually 
take place, it is difficult to see why it should be the exception 
rather than the rule, as the possibility of inclusion of sea water 
in fossiliferous limestone, owing to the decomposition of the softer 
parts of the organisms, would always be fairly great. 
Tracings of Serpulites dissolutus and of Arabellites sp. undet. 
have been identified in the limestones of the Lower Mottled division. 
It might naturally be suggested that the mottled effect in the lime- 
stones is due to actual castings of annelids, or to subsequent 
infillings of their borings. A well-known instance of the preserva- 
tion, on a Jarge scale, of annelid borings is that shown in the Middle 
and Upper Cambrian of the northwest Highlands of Scotland. 
In the Serpulite Grits and so-called “‘ Fucoidal”’ beds, large trumpet- 
shaped depressions are found on the surface, which lead downward 
into vertical, cylindrical tube structures, much constricted in 
places. These are without much doubt the castings of annelids, 
the sudden constrictions and widenings representing the peristaltic 
movements of the intestines. Overlying these beds, limestones 
and dolomites 1,500 ft. thick are found, consisting in part of mottled 
beds (the “mottled” or ‘‘Leopard”’ stone of the Sailmohr group). 
Throughout the whole series fossils are rare, and the limestones, 
which contain numerous cherts, are attributed to the calcareous 
and siliceous remains of the plankton. The mottling of the lime- 
stone is due to the fact that the worm castings are dolomitized, and 
darker than the rest of the stone. According to Peach,’ an explana- 
tion might be sought in the assumption either that the annelids 
were selective in their food, or that their gastric juices predisposed 
to dolomitization. 
On contrasting the markings of the Upper Mottled with these, 
one finds two points of difference. In the limestones under dis- 
cussion the markings are horizontally elongated, and irregular; 
t Peach and Horne, and others, The Northwest Highlands of Scotland, p. 380. 
