624 A TREATISE ON METAMORPHISM. 



along- the Republic trough. However, the magnetite thus deposited in 

 solid bodies is entirely subordinate to the vast amount which is dissemi- 

 nated through the rocks of the adjacent formations — the quartzites, slates, 

 and tuffs. Extensive deposition of magnetite is illustrated by the Michi- 

 gamme schist of the Marquette district and by the Hemlock volcanic forma- 

 tion of the Crystal Falls district." 



It will be remembered that hematite and limonite are very abundantly 

 precipitated in the belt of weathering as well as in the belt of cementation, 

 but magnetite very rarely forms in the belt of weathering. It therefore 

 appears that magnetite cement is especially characteristic of the belt of 

 cementation. 



Aluminum oxides. — The aluminum oxides which are deposited as cementing 

 substances are gibbsite, diaspore, and perhaps corundum. Both of the 

 hydrous minerals are known to be rather frequent cementing constituents 

 where the rocks are of an intermediate or basic character. Usually they 

 have not been regarded as important cementing minerals, but I suspect 

 they are rather more plentiful than has been supposed. My reason for 

 this belief is the frequency with which aluminum hydroxide is a by-product 

 in the common alterations of such minerals as the feldspars, feldspathoids, 

 micas, and other minerals. (See pp. 375-394.) 



CARBONATES. 



The important carbonate cements are calcite, dolomite, and siderite. 



caicite and dolomite. — Calcite and dolomite are treated together, since without 

 chemical analyses it is usually impracticable to discriminate between the 

 two where deposited in rocks. Also, there are various gradations between 

 calcite and dolomite. In general, it may be said that these minerals are 

 deposited in all the kinds of openings in rocks. They are more likely to be 

 extensively deposited in the calcareous rocks than in the siliceous rocks, since 

 in such rocks the solutions are sure to contain abundant carbonates, and 

 there are crystals of calcite and dolomite to serve as nuclei for deposition. 

 The calcium carbonate and magnesium carbonate may be deposited upon 

 calcite and dolomite, respectively, thus enlarging these minerals precisely 



« Van Hise, 0. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan: Mon. 

 U. S. Geol. Survey, vol. 28, 1897, pp. 444-452. Clements, J. Morgan, and Smyth, H. L., The Crystal 

 Falls iron-bearing district of Michigan: Mon. U. S. Geol. Survey, vol. 36, 1S99, pp. 150-152. 



