BRANNER: THE STONE REEFS OF BRAZIL. 119 
that the spaces between the sand grains are filled with carbonate of 
lime, and sometimes with a little hydroxide of iron. At a few places 
the hardening is caused by iron, but these instances are purely local. 
It is pointed out in the chapter upon the coral reefs that, when dead 
coral skeletons are left for a long while saturated with sea-water, some 
of the lime is replaced by magnesia and that ultimately a dolomite or a 
dolomitic limestone is formed. It is not known at present whether a 
similar change takes place in the binding material of the rock of the 
sandstone reefs. It may be that the original binding is what we find it 
to be at present, namely, a dolomitic lime rock, or it may be that the 
original binding material is carbonate of lime, which is partly replaced 
by magnesium carbonate from the sea-water. 
Tus Microscopic EXAMINATION OF THIN SECTIONS OF SANDSTONE FROM 
tun Rio Formoso Reers.! 
Thin sections of these sandstones show in general, under the micro- 
scope, a rock made up of irregular grains of quartz and organic fragments 
cemented together with calcite. (Nos. 1, 2, 3, 5, 6.) 
The proportion of quartz to organic fragments varies within consider- 
able limits in different sections. Besides the quartz, there are a few 
irregular sections of a feldspar, probably orthoclase, as well as some 
brown matter, which appears to be hydroxide of iron. Many of the 
quartz grains are grown through with fine dark and light colored needles, 
which may be rutile. Bright, strongly refracting, brilliantly polarizing 
little crystals both in the quartz grains and in the cement between 
them are believed to be zircons. Finally, there are in some of the 
sections bright yellow irregular fragments the nature of which was not 
determined, 
The only mineral requiring a detailed description for the purpose in 
hand, which is to find out, if possible, something in regard to the origin 
of the material from which the sandstones were formed, is the quartz. 
The quartz in thin sections occurs in large and small grains. An 
apparently continuous large grain breaks out often under crossed nicols 
into an aggregate of small grains showing different orientation, Some 
of the large grains are cracked and filled in along the cracks, sometimes 
with calcite and sometimes with hydroxide of iron. Most of the quartz 
grains are quite full of inclusions. In some cases, a grain is grown 
1 For this microscopic examination I am indebted to Dr. В. N. Brackett, of 
Clemson College, S. C. 
