22 THE PROCEEDINGS OF THE LINNEAN SOCIETY 



It is chiefly of the beds exposed for three or four miles along 

 the course of the Yass river after it reaches the town of Yass 

 that I shall at present treat. 



The Yass river enters the town of Yass on the east, then makes 

 generally a westerly direction flowing alike over hard and soft 

 rock, porphyry and shale, just as the dislocation of the strata had 

 marked out for it a course, which it has deepened and widened as 

 best it could. 



Very interesting are the cliff's on each side. Now we have two 

 hills of porphyry of very different composition, facing each other 

 — the junction of the two porphyries being the bed of the river — 

 then seventy feet of shale and limestone, every vertical foot of 

 which will yield a rich harvest to the geologist. A little further 

 and we have the section of a hill in which the strata broken off" 

 on each side are bent in towards the centre, making there almost as 

 acute an angle as the letter V. Again, a little further, and the 

 strata are reversed, vertical, then contorted in the most varied 

 curves, and the former impure limestone changed into marble, 

 marked with pink and other colored stains in patterns of the 

 sections of the shells and corals it formerly contained. 



The main course of the valleys on either side show a somewhat 

 different origin to that of the river. They are formed chiefly by 

 the scooping out of the softer strata, leaving ridges on each side 

 capped by the more indestructible rock. As the direction of the 

 dip of the strata is from 20° to 40° south of west, these ridges 

 that flank the valleys present to view in many places as you 

 turn east steep encarpments, and gentle slopes as you look 

 west. They bend round the igneous rock to the west of Yass, 

 forming part of a great curve, not, however, by an uniform 

 sweep, but by jumps, wrenched aside with a sudden twist and 

 interrupted by faults. These faults and twists have given rise 

 to smaller valleys and water-courses, which, in general, mark the 

 limit of the broken and intruded strata. 



Wonderful things are those hard rocks that cap the 

 Humewood and Belle Vale ridges. In one place we have an 

 ancient Coral Reef, rich in the most varied Palaeozoic forms, 



