BY SYDNEY B. J. SKERTCHLY. 61 
21. The Lower Cretaceous are characterised by thick 
shales and sandstones, but the important division to us is a 
remarkable bed described and named by Dr. R. L. Jack, 
the Blythesdale Braystone, whose boundaries have been 
traced by my old colleagues, Dr. Jack and Mr. Gibb-Maitland, 
for hundreds of miles along the eastern flank of the Cretaceous 
area. 
22. This Braystone, which is as porous as sponge, is 
the basement bed of the Lower Cretaceous, and being 
exposed by the excessive denudation of the overlying 
Desert Sandstone, it lies like a catchment-deain along the 
flanks of the highlands, and drains away the water which 
would otherwise flow as surface streams over the (in con- 
sequence) arid lands to the west. This is a most important 
factor in our argument. 
23. Upon the gently sloping Lower Cretaceous beds, the 
Upper Cretaceous rocks have been laid down unconformably. 
Their important member is the Desert Sandstone, whose 
isolated patches cap the low hills and make them remark- 
ably similar to the Koppies of the South African veldt. 
The Desert Sandstone marks a late stage in the shoaling 
of the Opal Sea, but it is most noticeable for our purpose 
from the great quantity of colloidal silica it contains. 
24. It is this colloidal silica which has given to Queens- 
land and New South Wales their treasures of Opal, and hence 
I call the waters in which the silica was dissolved the Opal 
Sea. Only a minute proportion of the colloidal silica has 
been converted into precious opal, the mass remaining In 
the normal form, much of it in the so-called Porcelainite. 
25. The country around the Desert Sandstone koppies 
is strewn with flakes of this porcelainite, bke shards upon 
the Euphrates’ plains. But the material is not baked clay 
at all. It extends over some thousands of square miles, 
and many miles away from the basalts: moreover, our 
basalts have very little baking power—they scarcely modify 
even the lignite upon which they sometimes repose. Again, 
the rock is not aluminous, but consists almost entirely of an 
admixture of colloidal and crystalloidal silica. 
26. I believe this so-called Porcelainite to owe its 
origin to the effect of solar heat upon the exposed material 
—that, in fact, it like the fauna and flora, is the product of 
