70 



THE GEOLOGIST. 



Before leaving the stratified rocks, allusion may be made to the 

 illustration they afiford of changes of physical condition and of oscil- 

 lations of leTel. Taking the coal in connexion with the limestone, 

 there is evidence of not less than fourteen changes of level ; as many 

 times, duiing 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 nins across 

 the strata from south by east to north by west : and a branch fi'om it 

 forks off to the north-north-west. The vein seems too small to be 

 worked with advantage. Its position gives probabiHty to the theory 

 that the igneous agency which forced upward the basalt produced also, 

 by sublimation, the ore which is found in the vein. 



"\^"hen 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 mns 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 mdely 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 Fame Islands. 

 The adjacent strata are very slightly altered in position j but their 

 structural charactei-s 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. 



