bkanxer: the stone reefs of brazil. 173 



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 hme 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. 



The Microscopic Examination of thin Sections of Sandstone from 

 THE Rio Formoso Reefs.^ 



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. (Xos. 1, 2, 3, 5, 6.) 



The proportion of quartz to organic fragments varies within consider- 

 able limits in diifereut 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. Fiiially, there are in some of the 

 sections bright yellow iiregular 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 oricrin 

 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 diflTerent orientation. Some 

 of the large grams 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 tl.is microscopic examination I am indebted to Dr. R. N. Brackett of 

 Clemson College, S. C. «'-».cii. vi 



