70 
THE GEOLOGIST. 
Before leaving the stratified rocks, allusion may be made to the 
illustration they afford of changes of physical condition and of oscil- 
lations of level. Taking the coal in connexion with the limestone, 
there is evidence of not less than fourteen changes of level ; as many 
times, during the period when these rocks were being deposited, the 
district was clothed with an abundant and marvellous vegetation, 
— as many times were there alternations of swamps and lakes, of 
estuaries and lagoons, and of seas sometimes profound, though gene- 
rally of moderate depth. 
A little northward of the basaltic dyke, a narrow crack or fissure of 
the sandstone contains galena or sulphuret of lead. It runs across 
the strata from south by east to north by west ; and a branch from it 
forks off to the north-north-west. The vein seems too small to be 
worked with advantage. Its position gives probability to the theory 
that the igneous agency which forced upward the basalt produced also, 
by sublimation, the ore which is found in the vein. 
When viewed from the shore near to Dunstan Square, this basaltic 
dyke, even to one unacquainted with geological principles, is a 
striking and interesting object. It rises perpendicularly through the 
stratified rocks, and runs in a direct line from west 85° south to east 
85° north. Its width is twenty-five feet, eontracting seaward to 
twenty feet. It stands in some parts ten feet above the strata, and 
appears like a wall rudely piled up by Cyclopean builders ; and, 
although in other parts it is broken down by the waves, its course can 
be distinctly traced for a considerable distance into the sea. The 
basalt is of the usual composition, augite and felspar ; but it is finer 
grained than the larger masses at Ratcheugh and the Farne Islands. 
The adjacent strata are veiy slightly altered in position ; but their 
structural characters are changed. Coal for some distance from it is 
valueless ; limestone near it will not burn into lime ; and shale and 
sandstone are indurated. At the point of contact, sandstones, shales, 
and limestones are much jointed and fissured, and assume the external 
form of basalt ; on the other hand, the basalt itself becomes cal- 
careous and siliceous. This transference of qualities and the struc- 
tural changes superinduced are the results of the igneous agency 
which, by its upward pressure, rent asunder the vast mass of stratified 
rocks, and then poured the molten basalt into the fissures. 
