Geological Society of London. 43 
The remarkable alteration of the limestone of Strath into a white 
saccharoid marble, first described by Maculloch, has hitherto been 
regarded as an instance of contact-metamorphism in a rock of Liassic 
age. The various writers who have described the geology of the 
district have followed Macculloch in classing the whole of the 
ordinary and altered limestone with the Secondary series of the Inner 
Hebrides. The author, however, saw reason in 1861 to suspect that 
some part of the limestone must be of the age of the Durness Lime- 
stone of Sutherland, that is, Lower Silurian; and he expressed this 
suspicion in a joint paper by the late Sir R. I. Murchison and him- 
self, published in the 18th volume of the Quarterly Journal of the 
Society. He has recently returned to the subject, and now offers 
lithological, stratigraphical, and paleontological evidence that the 
altered limestone is not Lias, but Lower Silurian. 
In lithological characters the limestone, where not immediately 
affected by the intrusion of the eruptive rocks, closely resembles the 
well-known limestones of the west of Sutherland and Ross-shire. It 
is not more altered than Paleozoic limestones usually are. It con- 
tains abundant black chert-concretions and nodules, which project 
from the weathered surfaces of the rock exactly as they do at Durness. 
These cherts do not occur in any of the undoubted Lias limestones 
of the shore-sections. The limestone lies in beds, which, however, 
are not nearly so distinct as those of the Lias, and have none of the 
interstratifications of dark sandy shale so conspicuous in the true 
Liassic series. 
The stratigraphy of the altered limestone likewise marks it off from 
the Lias. There appears to be a lower group of dark limestones full 
of black cherts, and a higher group of white limestones with little 
or no chert, which may be compared with the two lower groups of 
the Durness Limestone. A further point of connexion between the 
rocks of the two localities is the occurrence of white quartzite in 
association with the limestone at several places in Strath, and of 
representatives of the well-known ‘fucoid beds” at Ord, in Sleat. 
These latter strata form a persistent band between the base of the 
limestone and the top of the quartzite, which may be traced all the 
way from the extreme north of Sutherland southward into Skye. 
Paleontological evidence confirms and completes the proof that 
the limestone is of Lower Silurian age. The author has obtained 
from the limestone of Ben Suardal, near Broadford, a number of 
fossils which are specifically identical with those in the Durness 
Limestone, and so closely resemble them in lithological aspect that 
the whole might be believed to have come from the same crag. 
Among the fossils are species of Cyclonema, Murchisonia, Maclurea, 
Orthoceras, and Piloceras. 
The relations of the limestones containing these fossils to the other 
rocks were traced by the author. He showed that the Lias rests upon 
the Silurian limestone with a strong unconformability, and contains 
at its base a coarse breccia or conglomerate, chiefly composed of 
pieces of Silurian limestone, with fragments of chert and quartzite. 
~The metamorphism for which Strath has been so long noted is con- 
