THE VICTORIAN NATURALIST. 121 



resembles the rock shown commonly to the west of Melbourne. 

 In some places, however, the lava has filled in old valleys formed 

 by the ancient streams, and where these old river courses have 

 been cut across by the present stream we see, in favourable spots, 

 phenomena such as are displayed at the present locality. As the 

 river bed is still occupied in places by basalt which is in its 

 natural position, it is clear that the Saltwater River is flowing here 

 at a higher level than it was before the lava outflow, and it will 

 also be noticed that fully 200 feet in thickness of lava is exposed 

 in section. 



As will be seen from the photographs the most peculiar thing 

 about the basalt where its thickness is greatest is the fact that it 

 is divided into columns. In the present case single columns are 

 shown 60 feet in height and about 18 inches in diameter. Where 

 the stream strikes the base of the cliff, so that a good fresh face 

 is maintained, the columns are as clean cut as with a saw. In 

 other places, where the columns are not so freshly exposed, they 

 are cut into short lengths by transverse joints, and the slightly 

 weathered blocks look like piles of cheeses. The un weathered 

 columns are marked by transverse banding, so that, as was 

 suggested by Mr. G. Sweet, F.G.S., with whom I visited the 

 section early in the present year, they look like piles of earthen 

 tiles which have become fritted together by overheating in a kiln. 



Columnar structure is common in igneous rocks, and its cause is 

 not difficult to seek. To take die present case — basalt is a rock 

 made up of a number of interlacing crystals, the nature of which 

 we need not at present consider. In the molten condition the 

 different minerals are mixed into practically a homogeneous mass. 

 As cooling takes place these minerals separate out from the 

 common mass, and as crystallization is accompanied by diminution 

 in bulk, a shrinking takes place in the rock. Besides this, there 

 is the simpler contraction caused by mere cooling. It is owing to 

 the shrinking caused in these two ways that the divisional planes 

 are produced. If the sheet is thin and exposed on its upper 

 surface to the atmosphere the cooling is comparatively rapid, and 

 the divisional planes are irregular. In thicker sheets, especially 

 at the base of the flow, and in dykes, the divisional planes are 

 usually more regularly developed, and a columnar structure arises. 

 The transverse joints in the columns are produced by the same 

 cause. 



Owing to the way in which the columns are formed, they stand 

 at right angles to the cooling surface, which in the present case 

 is formed by the sides of the valley trough in which the lava 

 flowed. Some of the columns in the photographs are seen to be 

 vertical, while others are horizontal where they abut on the steep 

 sides of the old valley. In some cases, owing to the fact that the 

 present stream cuts the course of the old one obliquely, the ends 



