1884.] 
Geological Structure of the Sahara. 541 
the temperature of eradiation has risen from 21*5 to 36*8 per 
cent of its present value. 
As the mass of Sirius is 13-8 times that of the Sun, Sirius 
must have passed through the same period of development 
in 2040 years, and during this lapse of time its temperature 
of eradiation must increase from 80 to 137 per cent of that 
temperature. If we therefore assume that the radius of 
Sirius is twenty times greater than it would be in the present 
state of density of the Sun, or that the present mean density 
of Sirius is the 8000th part of the mean density of the Sun, 
it would follow that the eradiation temperature of Sirius 
2040 years ago was 20 per cent smaller, and is now 37 per 
cent greater, than the present temperature of eradiation of 
the sun. As the light of the sun is at present yellowish 
white, it is very easily conceivable that a temperature 
of eradiation 20 per cent lower might correspond to a reddish 
colour, and one 37 per cent higher to a blueish white. 
According to this hypothesis the diameter of Sirius must 
at present amount to 9 million miles, giving, at an assumed 
distance of 20 billion miles, an apparent diameter of 
0*09 second. It would follow also from the hypothesis that 
the temperature of Sirius is still increasing, and has only 
reached about 36*8 per cent of its maximum value. Hence 
the blue colour of the light of Sirius may considerably 
increase in the future. 
VII. THE GEOLOGICAL STRUCTURE OF THE 
SAHARA. 
S R. K. A. ZITTEL has published the following fadts 
and conclusions as the preliminary result of his ex- 
plorations in the Libyan Desert : — 
The Sahara is distinguished by an exceedingly simple 
geological structure, by the horizontal position of most of 
its sedimentary rocks, and by the absence of faults. To 
the southern slope of the Atlas in Morocco, which forms 
the northern boundary of the Sahara, there are joined 
palaeozoic formations (carboniferous and Devonian), upon 
