86 Armitage, Country about West Essendon. [^'*^Sepu^^' 



altered Older Basalt is to be seen in a road cutting near the 

 post marked " E. T. B." Among the alteration products of 

 this basalt may be noted concretions of limonite, hematite, 

 psilomelane, and, most interesting of all, as showing the presence 

 of the element cobalt in our Older Basalts, black concretions of 

 the mineral asbolite, a hydrous oxide of cobalt and manganese. 

 Other exposures of the Older Basalt are to be noticed along the 

 Saltwater River at the Bend, from stream level to varying 

 heights above it. A feature of these exposures is the variable 

 stages of weathering the basalt has undergone in different parts. 

 In some places it has weathered into a red earth like laterite, in 

 others to a soft white wackenite, while in one place a little to 

 the north of the Sand Pits it may be seen on the riverside in as 

 fresh and unweathered a condition as the freshest of the Newer 

 Basalts. Here it is a dense, fine-grained, tough, dark-blue rock, 

 showing rubbly and finely columnar jointing (see Petrographical 

 Note, No. i). At one point a peculiar egg-shaped structure (about 

 10 feet in long diameter) of unweathered basalt contains basalt 

 considerably decomposed. The surface of this Older Basalt is 

 everywhere very much eroded. Its probable extent has been 

 discussed by Mr. T. S. Hart (3). The rock on which it rests is not 

 visible anywhere in the vicinity, but it is probable that it is 

 the usual Melbournian (Silurian) shales and sandstones, which 

 constitute so much of the bed-rock of the Melbourne district. 



The Newer Volcanic Basalt. 



The Newer Basalt of West Essendon is, for the most part, a 

 mere skin lying on the Tertiary sediments underneath. It is 

 portion of the eastern edge of the vast lava sheets constituting 

 the Keilor Plains, which are continuous with those stretching 

 away into the fertile Western District through the great valley 

 of Victoria. In the aggregate, these Newer Volcanic basalt sheets 

 cover an area estimated at between 9,000 and 10,000 square 

 miles, and are generally accepted as comprising the third largest 

 area of such rocks in the world. The Deccan Plateau, in India, 

 is partly covered with a lava sheet over 200,000 square miles in 

 area, and up to 6,000 feet in thickness (4). In North- Western 

 United States, covering Idaho, Oregon, and Washington, to an 

 extent of over 100,000 square miles, to a depth of several 

 hundred feet (5), and, according to Dr. E. Hull (4), in some places 

 2,000 to 3,000 feet in thickness, exists another expansive vol- 

 canic plain. But there are also extensive volcanic sheets of 

 doubtful age at the top of the Stormberg series (Upper Triassic) 

 in Southern Cape Colony. Of these. Dr. A. H. Green (6) writes : 

 — " Over an area certainly 120,000 square miles in extent, and 

 probably much larger, dykes, sheets, and huge masses of trap 

 meet one at every turn." According to Dr. F. H. Hatch (7), 



