316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
show that an extensive low-level glaciation occurred in Cambrian 
times in low latitudes in South Australia; ¢ indeed, it seems probable 
that, in spite of many great local variations, the average climate of the 
whole world has remained fairly constant throughout geological time. 
Whereas it has often been represented, in accordance with the nebular 
theory, that volcanic action has steadily waned, owing to the lowering 
of the earth’s internal fires and the constant thickening of its crust, 
yet epochs of intense volcanic action have recurred throughout the 
world’s history, separated by periods of comparative quiescence. 
Whereas it has been assumed, as a corollary to the nebular theory, 
that the force which uplifted mountain chains was the crumpling of 
the crust owing to the contraction of the internal mass, yet observation 
reveals that the crust has been corrugated, and fold mountains formed 
by contraction to an extent far greater than secular cooling can 
explain. 
2. The materials of the inner earth——This planetismal hypothesis 
is not only consistent with geological records, but also with the known 
facts as to the internal composition of the earth and the structure of 
extra-terrestrial bodies as revealed by meteorites. Meteorites are of 
two main kinds—the meteoric irons, which consist of nickel iron, and 
stony meteorites, which are composed of basic minerals. Some of the 
stony meteorites have been shattered into fault breccias, showing that 
they are fragments of larger bodies which were subject to internal 
movements, like those that have formed crush conglomerates in the 
crust of the earth. Those stony meteorites, therefore, both in com- 
position and structure resemble the rocks in the comparatively shallow 
fracture zone of the earth’s crust. The nickel-iron meteorites, on the 
other hand, represent the barysphere beneath the crust. 
The earth appears to consist of material similar to that of the two 
types of meteorites; but whether the proportions of the two materials 
in the earth represent their proportions in other bodies and in meteoric 
swarms is problematical. There appear to be no satisfactory data for 
an estimate of the relative abundance in space of the iron and stony 
meteoric material. Stony meteorites have been seen to fall far more 
frequently than iron meteorites; but the largest known meteorites 
are of the nickel-iron group, although this material, in moist climates, 
very soon decays. The most reliable indication as to the relative 
amounts of the stony and nickel-iron meteorites is given by a com- 
parison of the weight of the two types of material in meteorites of 
which the fall was seen. According to Mr. Fletcher’s list of the 
meteorites in the British Museum up to 1904, the collection included 
319 specimens of which the fall is recorded: of them 305 specimens 
were stony meteorites of an average weight of 2.63 pounds, 9 were iron 
«As shown by the work of Professor Howchin, of Adelaide. 
