24 THE PROCEEDINGS OF THE LINNEAN SOCIETY 



micaceous and gritty, with an occasional thin band of limestone 

 until they pass into hard compact grit, at a thickness of about 

 500 feet from the Porphyry. The upper and lower beds of grit 

 are separated by strata of greenish shale ; the lower grit is ia 

 some parts thin, flaggy, and easily disintegrated, in others 

 compact and rudely jointed. 



The lower portion of the grit is in some places full of cubical 

 crystals of oxide of iron. Some of the upper grit has been 

 quarried for building. It exhibits, in many places, distinct ripple 

 marks. The top course is, however, very hard, siliceous, coarse- 

 grained, and sometimes almost conglomerate, with signs of 

 altered condition. I have obtained no fossils from the grit. 



The top of this upper bed affords a convenient means of 

 dividing the Yass Beds into two parts, giving to the lower 

 portions a thickness of about 700 feet. 



The next division, the especially fossil bearing half, is best 

 studied by starting from the rock just described at a point where 

 it crosses the river, about a quarter of a mile further down. The 

 strata, after some thin, not very coherent, gritty beds, gradually 

 become calcareous, until they pass into a compact flaggy 

 limestone, just above the Spirifer Beds. The fossils found lowest 

 in this division were a Lingula and a Troclius, succeeded 

 occasionally by an Ortlionotus, and some ribbed Spirifers, until 

 at a thickness of about 185 feet we have a small band of black 

 impure limestone, nine inches thick, loaded with fossils. 



This band at first contains chiefly several species of 

 Murcliisonia and some of Loxonema, succeeded by a layer of 

 Spirifers, and these by a thin mass of Pterinea and Modiolopsis. 

 Among the Spirifers we find here, however, there is no Spirifer 

 Yassensis. That Spirifer cannot be obtained nearer than the 

 Devonian of the Murrambidgee, in which strata, at a distance of 

 about twelve miles from Yass, it abounds. A Retzia, Orthis, and 

 Orthoceras, were also found here. 



I will now pass over some flaggy limestone ; two beds, from 

 3 to 4 feet thick, of compact sub-crystalline limestone, the latter 

 though full of fossils, yielding little ; and some Calcareous gritty 

 beds, to an impure limestone, from which many species have been 

 obtained. 



