390 SECTIONAL TRANSACTIONS.—C. 
The present author, assisted from time to time by Dr. Wilson, has made an 
extensive re-examination of the earlier surveys and completed much of the unfinished 
ground. Maps showing the results are exhibited, together with a number of the 
maps of Lapworth, Stacey Wilson and Andrew. 
The workable Manganese Ore bed has been found of great value in elucidating the 
detailed structure of the Harlech Dome, and Lapworth and Wilson’s statement 
that it occupies a single geological horizon is confirmed. J.G.Goodchild’s conclusion 
that there are two beds of ore separated by at least 850 feet of sediments appears to 
have been based chiefly on deceptive repetitions of one bed by strike faults. 
The folding, faulting and cleavage of the region are described, with their effect 
on the numerous igneous intrusions. The dykes are often displaced by an important 
system of faults that trend on the whole about N30°H. 
Mr. A. James and Prof. EK. W. Sxeats.—The Basaltic Ridges and Valleys 
of the Stony Rises near Colac, Victoria. 
The district lies west of Colac, which is 92 miles west of Melbourne. 
Topographic and geologic features are all basaltic, and geologically recent, con- 
sisting of ridges and valleys between 400- and 600-foot contours, hills and craters 
rising above the ridges and numerous lakes, both shallow and deep, whose margins 
approximate to the 400-foot contour line. 
Scores of thousands of ridges and valleys occur within an-area of roughly 20 square 
miles. The ridges vary in height from 18 inches to 50 or 60 feet, in length from 
30 yards to about half a mile. Characteristically hogbacked in shape, with side 
slopes from 30° to nearly vertical, many show longitudinal undulations, scme 
terminating in steep knolls. Longitudinally straight.and curved forms occur. Many 
ridges show gaping summits up to 20 feet in depth, and are known locally as breached 
barriers. Ina few cases the breach is occupied by dyke-like basalt of different texture. 
Others show depressions of the apex of the hogback amounting to from 1 to 5 or 6 feet. 
The basalt is usually fresh, with a smooth or vesicular surface, a general absence of 
soil, and with columnar jointing sometimes closely set, sometimes gaping, at right 
angles to the exterior slope of the ridge. Near the shores of Lake Corangamite the 
ridges are small, widely spaced, rising above a flat surface of bedded tuffs and the 
typical valleys are wanting. To the 8.W. away from L. Corangamite the ridges occur 
at a higher elevation, are confluent and rise 50-60 feet above valleys, which are 
original features constituting the space between the ridges, and are clearly not erosion 
features, for they show no constant longitudinal slope but considerable undulations 
in level. No surface flowing stream traverses the Stony Rises, but rainfall sinks 
below the surface and emerges at low points as strong, fresh-water springs. 
Hypotheses as to the origin of the ridges, knolls and valleys have been discussed 
in the field. 
They are not normal lava flows, do not proceed directly from any of the volcanic 
cones and two linear trends, N. and 8. and N.W. and §.E., can be distinguished. 
Sometimes one set alone occurs, but in the heart of the Stony Rises both sets often 
occur. They are believed to develop from thousands of small fissures beneath them, 
the rise of the basalt magma through each fissure and its limited superficial extrusion 
may be connected with pressure on a basaltic reservoir beneath, due to the sagging 
of the surface to which many of the lakes are due. Renewal of the sagging may cause 
further rise of basalt, causing the solidified crust of the ridge to form a gaping fissure 
by pressure, and in a few places to form a dyke in the gap. Most of the breached 
ridges away from Lake Corangamite, however, probably owe the development of 
the breach or sag to subsequent withdrawal of molten basalt below the crust. This 
withdrawal may be connected with relief of pressure beneath, developed by explosive 
activity, by which the scoria and tuff cones of Mt. Porndon and other volcanoes were 
formed. This view is supported by the presence of basalt caves near Mt. Porndon 
with a hogback floor, whose arched roofs bear pendent basalt stalactites indicating 
that withdrawal occurred while the basalt below the crust was still molten. 
