60 THE VICTORIAN NATURALIST. 



become rubbly, or so cut by joints that they break up into small 

 fragments when weathering. The dyke-stone itself is of yellowish- 

 brown colour, and is spotted with dark blotches which a lens will 

 show to be decomposed crystals. The dyke dies away towards 

 the top of the cliff, and in the road cutting above is visible only 

 as a band of spotted clay about a foot in thickness. Close by 

 this clay band is a beautiful example of a fault, which was very 

 conspicuous last year when the road was cut down. The strata 

 on each side of the fault were so very different in texture and 

 colour that the most unobservant could hardly fail to notice it. 



If we now walk down the valley towards the railway cutting to 

 the westward of the Zoological gardens, we cross over a deposit 

 of waterworn pebbles and clay, mixed up in a confused manner. 

 This is the alluvial deposit of the creek and its tributary gullies. 

 As we climb the hill towards the railway line, we find the soil 

 changing in colour to a reddish hue, and when we climb the 

 railway fence, in defiance of the caution to trespassers, and enter 

 the cutting, the cause is evident. The strata on top consists of 

 quartz gravel and sand, deeply stained with oxide of iron. 

 About half-way down the cutting we come on a mass of clay 

 which rises high on the south side of the bank. The gravel beds 

 rise up to rest on its flanks, and no trace of stratification is visible 

 in it. A close examination will show many small white spots of 

 a softer material than the rest of the mass. A few yards further 

 to the south and this deposit sinks below the level of the rails. 

 Close to the end of the cutting it again appears, with the same 

 white spots throughout its mass. As we trace it further along 

 nodules make their appearance in it. Some of these have many 

 concentric coats which flake off under the hammer, and in the 

 centre we find a very hard mass, which breaks with difficulty, and 

 which we find to be a dense black basalt. If we now re-examine 

 the bank carefully we see that many of the nodules are much 

 decomposed, and we can trace a series of them, from the dense 

 ones we first examined down to a mere concentric staining in the 

 spotted clay. Thus we see that the clay is merely the result of 

 the decomposition of the basalt in situ. Now that we have 

 learned the appearance of decomposed basalt, we can trace this 

 clay in many places ; for instance, a good section is shown o: % . 

 Flemington-road, near the old Powder Magazine, again near the 

 Pig Market, and the same clay is seen in the upper part of the 

 dyke we mentioned, further up the creek. 



Underneath the basalt, quite close to the semaphore, appears 

 a yellowish mass of clay. This is part of the same silurian rock 

 we saw to the northward. Here it formed a ridge which was 

 covered by the flow of basalt. 



The upper surface of the basalt was once level, or nearly so ; 

 but before the overlying deposits were laid down it had become 



