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 



