114 PROCEEDINGS OF SECTION C. 



pyiamids of quartz and large crystals of biotite mica are very 

 noticeable in these porphyries. Of a specimen from half a mile 

 below the railway weir Dr. Woolnough writes : — " A magnificent 

 quartz porphyry ; certainly a massive eruptive rock, and not a 

 tuff." 



Of another beautiful sample, from a paddock to the south and 

 next to the Roman Catholic cemetery, he writes : — " Quartz 

 porphyry, with very remarkable red felspars, the colour being due 

 to alteration, probably stilbite ; calcite and epidote are abundant 

 also." 



This porphyry, with a greenish-grey base, is highly ornamental, 

 with its red felspars, and when polished makes a most attractive 

 addition to the ornamental building stones of New South Wales. 



These massive bedded porphyries are overlaid to the west by 

 more tuffs. About a mile above Hatton's corner, on the left bank 

 of the Yass River, is to be seen a contact between the porphyry and 

 the tuff beds, which at this spot is in the form of a soft blue-grey 

 rock. Fissures in this tuff are filled with another cherty altered 

 tuff, which is found also to exist as an extensive bed in Marchmont 

 Paddock. This tuff in hand specimens has the appearance of a 

 radiolarian rock, the matrix being studded with innumerable light- 

 coloured spherical masses, having a close resemblance to radiolaria. 

 Dr. Woolnough, however, some years ago decided it was an altered 

 tuff with andalusite. 



This bed is overlaid by a layer of volcanic breccia, which is 

 well seen also in Marchmont Paddock. The breccia is succeeded by 

 a very hard, fine-grained rock of a blue-black colour, almost like 

 basalt in appearance. This rock, which Dr. Woolnough considers 

 to be a quartz porphyry tuff, is best examined at Booroo Ponds 

 Creek, about half a mile above its junction with the Yass River at 

 Hatton's Corner. It is remarkable for its distinct bedding and the 

 diamond-shaped blocks into which it splits. Occasional hexagonal 

 blocks are also to be seen. (PI. VI.) 



The tuff which has a dip to the S.W. of about 12° also forms a 

 bar across the Yass River and underlies the big limestone chff three- 

 quarters of a mile above Hatton's Corner. It forms the highest bed 

 of the No. 3 porphyry belt, and is overlaid by the fossiliferous 

 Hume beds. 



The Hume Beets'^. — These consist of a series of shales, sand- 

 stones, grits and limestones, of a thickness estimated to be well over 

 2,000 feet, and extending from the No. 3 porphyry at Hatton's 

 Corner to the other side of Bowning Hills. Being very much faulted 

 and folded, the thickness of the beds can only be stated approxi- 

 mately, especially as they vary in thickness in difierent localities not 

 far distant from one another. Mr. John Mitchell's paper on the 

 Bowning Beds, read before the first meeting of this Association, 

 deals with the Bowning section of the Hume Beds, of which a full 

 de.'icription would entail a very lengthy paper, so that a very brief 

 notice will necessarily be given here. At Hatton's Corner the 



1 Jenkins, C. : " Geclogv of Yass Plains" Proc. Linn. Soc. A'.S.IK., Vol. HI., 1878. 



