BY LEO A. COTTON. 235 



Geological. — The district for some miles round Guyra consists 

 of a basalt-formation. This was observed to extend for some 8 

 miles to the south, and 5 miles to the west. The basalt noticed 

 at Black Mountain contains phenocrysts of olivine, while in that 

 of Guyra no such crystals were observed. This may indicate a 

 dififerent origin for the two lavas mentioned. A few miles to the 

 south of Black Mountain, on the Armidale Road, the basalts were 

 observed to overlie unconformably a series of Palaeozoic sediments 

 consisting of quartzite, slate, and conglomerate. From aneroid 

 measurements it appears that the basalt here attains a thickness 

 of at least 1000 feet. These sediments were seen to be intruded 

 by granite, a few miles to the south of their junction with the 

 basalt. 



On the western side of the lagoon, a series of altered sediments 

 was found, at a distance of 6 miles from Guyra; while, at a dis- 

 tance of 7^ miles in the same direction, a very striking porphyry 

 was met with. This rock contains phenocrysts of orthoclase and 

 quartz, the former being so abundant as to give a pink color to 

 the rock. It was also observed some 20 miles north of this spot. 



Fig. 2 of Plate xxi., shows a diagrammatic section from north- 

 west to south-east, through the Guyra Lagoon. 



At the time when the writer visited the lagoon, the bed was 

 being used as pasture land. Local residents state that no con- 

 siderable amount of water has been present since 1902, but 

 that previous to that year the lagoon was never known to be 

 dry. Since 1902, small bodies of water have accumulated after 

 heavy rains, but have quickly disappeared. So little does this 

 basin now function as a lake, that a well sunk to a depth of 

 35 feet in the lake-bed failed to reach water. Locally it is 

 commonly believed that the lake has been exhausted by the drain 

 made upon the artesian waters in the western areas. The 

 geological structure of the district scarcely admits of such an 

 explanation. An obvious possible cause is a diminution in the 

 rainfall; and with a view of ascertaining whether this afforded 

 any explanation, the record of the rainfall of Guyra for the past 



