FACTORS PRODUCING COLUMNAR STRUCTURE IN 



LAVAS AND ITS OCCURRENCE NEAR 



MELBOURNE, AUSTRALIA 



ALBERT V. G. JAMES 



University of Melbourne 



The following notes on columnar structure owe their origin to 

 a study of the basalt in the Sydenham area, 15 miles northeast of 

 Melbourne, Australia. There, a young stream, Jackson's Creek, 

 has sunk its bed in the Upper Kainozoic lava flows to a depth of 

 250'. At some points the Silurian and Upper Ordovician sediments 

 are exposed in its bed but at many other places the stream has not 

 yet reached the level of the old valleys and a great thickness of 

 basalt is exposed to view. 



The best columns are found along Jackson's Creek in an area i J 

 miles in length. In it is a low scoria cone of the same age as tha 

 lava. Three miles to the north is a Hne of volcanoes from which the 

 later lava floods came. Of the columns the best perhaps are those 

 known as the Sydenham Organ Pipes. These are 102' high, are 

 perfectly developed, and give examples of the various structures 

 discussed in this short paper.' 



I. Cause of columnar structure. — The attitude, size, shape, and 

 regularity of the columns depend upon {a) the viscosity of the lava; 

 {h) the temperature of the lava; (c) the rate of cooling; {d) the 

 regularity of cooling; {e) the homogeneity of the lava. Shallow 

 surface flows are much disturbed by movement and are usually 

 distorted by folded-in masses of scoriaceous material, so that 

 homogeneity is lost. Cooling is irregular and columns do not 

 form. The importance of homogeneity as a factor in columnar 

 formation cannot be emphasized too much. The great controlHng 

 factor in the formation of columns is contraction due to cooHng. 

 The rock, on cooling, tends to contract but all solids have the 

 power to extend somewhat under tension. When the tension due 



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