172 



GEOLOGICAL NOTES— DERWENT VALLEY COUNTRY. 



not only the difficulty of getting in the drills deep 

 enough for effective blasting, but the rock is so unusu- 

 ally hard and splintery that there was no avoidance of 

 serious damage to face and hands in the subsequent 

 breaking up with the hammer. The depth of the cutting 

 at this point is about 14 feet. The diabase is rudely 

 •columnar, and resting upon it is a band of altered sand- 

 stone (Plate XIII. ), the section showing more conclusive 

 evidence of the presence of an intrusive sill than I have 

 seen elsewhere inland, though similar sections are com- 

 mon enough on the shores of Tasman's Peninsula, Bruni 

 Island, and the Channel. Towards the western end of 

 this cutting the sandstone is much dislocated bv the 

 lifting agency of the intrusive rock. About half a mile 

 farther on is a long cutting through altered mudstone, 

 the diabase onh' showing here and there. The general 

 dip is about E.S.E., and in this direction it will pass 

 under a neighbouring lofty hill of sandstone, which is 

 normally the next member in the ascending series. The 

 differences in level and the changes in the direction of 

 dip of the sedimentary rocks along the whole route show 

 that they have been much disturbed and faulted bv the 

 intrusive diabase, which everywhere underlies them at a 

 greater or less depth in the form of sills or laccolites. 



At 334 miles, at a sharp bend in the Russell Falls 

 River, is a fine section showing columnar diabase un- 

 derlying altered and much jointed mudstone. 



The diabase shows itself here and there for the next 

 mile, but is mostly hidden by sand and gravel, and the 

 waste of the mudstone which is the bed rock of this 

 part of the district. At S/4 niiles a cutting was taken 

 through mudstone of normal character, but with a 

 change of dip to S.W. The next cutting is through 

 mudstone at first in regular bedding, but towards the 

 Western end large loo&e angular blocks of the same rock 

 were met with, together with rounded boulders of 

 •quartzite and other ancient rocks, and occasionally of 

 diabase. One weathered block of the last named 

 measured 3 feet by 2 feet, with a thickness of about 7 

 inches. The next cutting is through soft sandstone 

 lying conformably to the mudstone. This is the last 

 appearance of the sedimentary rocks, and the terminus 

 ■of the line stands on sandy clays and gravel thinly 

 covering massive diabase. 



