Secondary Beds of Northern Australia. 123 



source from whence the sandy alluvial deposit of the plains 

 had been derived. 



Several of the shells brought down were broken from the 

 sandstone rocks, where they had formed layers of organic 

 remains on the bedding planes. 



Approaching the western edge of the plains, the M'Kinlay 

 Ranges are first seen at a distance of thirty to forty miles, 

 but the Silurian formation, of which they consist, extends 

 for some miles into the plains. The only portion examined 

 was a bight or hollow in the range, where it trends back to 

 the westward (the general direction being north and south), 

 and whence the drainage of a considerable area makes 

 its escape to the plains through the Cloncurry River and 

 its tributaries. This portion of the country is slightly un- 

 dulating, studded with detached hills, and crossed by low 

 ridges, with the rock here and there cropping above the 

 surface of the ground. Occasionally detached masses of 

 rock rise abruptly on the line of these ridges from twelve to 

 fifty feet in height — or peaks and oblong hills follow the 

 same lines. The country shows no sign of active denuda- 

 tion, and its present form is due to the slow disintegration 

 of the rock, and the gradual removal of the finer particles. 

 All the gravel observed was angular, both on the surface 

 and in a shaft sunk in the bed of a small creek when looking 

 for water. Rolled drift will probably be found in the bed of 

 the Cloncurry River and some of the larger streams. 



The detached masses of rock and small hills in most 

 instances consist of rock impregnated with iron, or changed 

 into quartzite. In one place a low rise was crested for 

 several hundred feet by a quartz vein from five to six feet 

 wide and from twelve to twenty feet high, cropping up 

 white and bare above the grass, with vertical sides and sharp 

 angles, giving little indications of wear. In these cases the 

 hardened rock or quartz had resisted the action of the 

 weather, while the softer rock decayed, and was gradually 

 washed away. 



To a certain extent the same may be said of the low 

 ridges — these consisting of ironstone, quartzite, and in some 

 places a semi-granitic rock. The constituents of the latter 

 form of rock seemed to be collecting into separate bodies, so 

 as to resemble a coarse granite ; and in the weathering of 

 the rock the softer portions had been worn away, leaving 

 the harder parts projecting on the surface, and rounded by 

 exposure, so as to resemble a conglomerate, for which it was 



