1848.] DAWSON ON COLOURING MATTER OF RED SANDSTONE. 27 



render it probable that it originated from the action of free sulphuric 

 acid on the calcareous matter previously accumulated in the seas of 

 the period. This view must also be extended to the anhydrite, which 

 occurs in layers associated with the common gypsum, since its rela- 

 tions entirely preclude the suppositions that it can be gypsum altered 

 by heat, or that it can have been produced by acid vapours passing 

 through limestone ; I am not however aware under what circumstances 

 anhydrite could be chemically deposited from water. In the series 

 of formations found in Nova Scotia, gypsum as well as red sandstone 

 appears for the first time in the lower carboniferous series, and it will 

 soon appear that this simultaneous development on a great scale of 

 red oxide of iron and sulphuric acid may not be accidental. 



We may next endeavour to ascertain the sources from which the 

 materials of the rocks above- noticed have been derived. In the car- 

 boniferous period, the Silurian, metamorphic and hypogene rocks 

 seem already in Nova Scotia to have formed ridges traversing and 

 separating the basins in which the newer strata were deposited, and 

 furnishing large quantities of detritus which can easily be recognised 

 in the carboniferous conglomerates, sandstones and shales, and indeed 

 constitutes the mass of these beds. The limestones have evidently 

 resulted from the growth of shells and corals i7i situ ; and the gypsum 

 is also of local origin, since it can scarcely be supposed that, at the 

 period of its formation, the sea was charged either with sulphuric 

 acid or sulphate of lime, over wide areas, while it is highly probable 

 that these substances, if brought from the land or the bottom of the 

 sea, would produce beds of gypsum in the vicinity of the places whence 

 they were derived. It thus appears that the materials of the lower 

 carboniferous rocks have in general been obtained from the older 

 formations the remains of which are still seen in their vicinity, and 

 we may therefore expect to find, in the same older formations, the 

 sources of the red colouring matter. If in accordance with this view 

 we examine the Silurian and metamorphic rocks, it at once becomes 

 apparent that the red oxide of iron cannot be attributed to the de- 

 gradation of red-coloured rocks, since these form a very trifling pro- 

 portion of the older formations. Neither can this colouring matter 

 be attributed to the mechanical trituration of iron ores, since though 

 large deposits of specular iron ore exist in the Silurian system, this 

 mineral is too hard and intractable to have furnished the finely-divided 

 colouring matter of the red sandstones and shales. It is also 

 worthy of notice, in reference to this iron ore of the Silurian system, 

 that the greater part of it occurs in the form of thick beds, abounding 

 in fossil shells, and which seem to have been produced by the depo- 

 sition of iron ore in the state of sand or scales derived from the waste 

 of older deposits ; it cannot therefore have been, at the time of the 

 formation of the carboniferous strata, in a state very different from 

 that in which it is at present found. The remainder of the peroxide 

 of iron of the Silurian system occurs in irregular veins traversing 

 altered rocks, and is generally crystalline, though in some places ac- 

 companied by earthy red ore capable of having acted as a colouring 

 matter. The only other form in which large quantities of iron occur 



