386 Proceedings of the Royal Physical Society* 
Silurian Rocks. 
The Silurian rocks in this district are remarkably similar 
in their composition and structure. There are no alternations 
of limestones, sandstones, shales, &c., as in the newer forma- 
tions, or even in the Silurians of some other districts. The 
rocks are almost entirely composed of water-worn quartz 
grains and clay. These materials have been deposited either 
as mud, or as sand of different degrees of fineness, or of a 
mixture of both, and have produced shales or sandstones, 
which are extremely indurated, from the long continued meta- 
morphic action to which they have been subjected. The ar- 
gillaceous sandstone or greywacke rock is the predominant 
rock. It is generally fine grained, and of a uniform greyish 
colour, breaking with an uneven and somewhat conchoidal 
fracture. The beds in which it occurs are generally of con- 
siderable thickness, though occasionally it is met with in thin 
layers, as at the Common Craig, where it has for long been 
quarried as a building stone. It forms a beautiful and very 
durable building material. A large portion of Moffat is built 
of it. The shales are generally light coloured, but in some 
places they are more or less blackened by anthracite, derived 
originally either from the animal bodies whose remains abound 
in these shales, or more probably from the sea-weeds that 
were their companions in the Silurian seas, and which, from 
their soft cellular structure, have left no definite traces behind 
them on the rocks. Iron pyrites, aggregated in small nodules, 
or scattered through the substance of the rock, is common in 
the shales. So also is alum ; at Garple the shales are so 
highly aluminous, that a mineral water is obtained by dashing 
the water of the burn against the more friable layers. Hand 
specimens of the shale are frequently broken up in the cabinet 
by the contained alum forming into masses of beautiful aci- 
cular crystals and separating the original layers of the clay. 
The whole of the strata are singularly free from lime. The 
only calcareous rock that I am acquainted with is found in 
Garple Glen, but it contains such a small proportion of lime, 
joined to a large amount of mud, that it is of no economical 
value. Small veins of calcareous spar, as well as of quartz, 
occur frequently in the rocks. The indurated light-greyish 
