SPECIFIC GRAVITIES OF MINERALS IN THE ZONES. 183 



in the first zone is, on the average, to produce minerals of lower specific 

 gravity than the original minerals, while in the deeper-seated zone the 

 tendency, on the average, is to produce minerals of higher specific gravity. 



Illustrations of the first rule are the minerals produced by the disinte- 

 gration and decomposition of rocks near the surface, out of which the sedi- 

 mentary rocks are built, Some of these are kaolinite (sp. gr. 2.6-2.63), 

 quartz (sp. gr. 2.65), calcite (sp. gr. 2.72), chlorite (sp. gr. 2.60-2.96), 

 serpentine (sp. gr. 2.5-2.65), talc (sp. gr. 2.7-2.8), zeolite (sp. gr. 2-2.4), 

 limonite (sp. gr. 3.5-3.96), etc. All of these minerals and most of the other 

 abundant undecomposed minerals, such as feldspar (sp. gr. 2.55-2.75), 

 which make up great masses of sedimentary rocks, have comparatively 

 low specific gravities. 



The second rule is illustrated by the change from low to high specific 

 gravity of the minerals where the sedimentary rocks are metamorphosed. 

 As just seen, the minerals which compose the unaltered sedimentary rocks 

 are originally those of low specific gravity. Some of the abundant result- 

 ant minerals in the equivalent metamorphosed rocks have considerably 

 higher specific gravities, as, for instance, muscovite (sp. gr. 2.76-3), biotite 

 (sp. gr. 2.7-3.1), pyroxene (sp. gr. 3.2-3.6), and amphibole (sp. gr. 2.9-3.4), 

 and the still heavier minerals, garnet (sp! gr. 3.15-4.3), staurolite (sp. gr. 

 3.65-3.75), chloritoicl (sp. gr. 3.52-3.57), hematite (sp. gr. 4.9-5.3), and 

 magnetite (sp. gr. 5.168-5.180). Less common heavy minerals are andalusite 

 (sp. gr. 3.16-3.2), fibrolite (sp. gr. 3.23-3.24), and chondrodite (sp. gr. 3.118- 

 3.24). With the above are the lighter minerals, quartz and feldspar; but 

 even these are quite as heavy as the average of the original minerals. 



It is noticeable in the altered rocks that in proportion as deep-seated 

 metamorphism is advanced the heavier of the above minerals appear. In 

 the early stages of the metamorphism of shales, mica develops plentifully, 

 and the rocks become slates. Where the metamorphism is more intense 

 the heavier minerals, garnet and staurolite, appear, the material of the pre- 

 viously developed micas being absorbed at the places occupied by the 

 garnet and staurolite. 



The garnet-, staurolite-, chloritoicl-, andalusite-, and tourmaline- bearing 

 schists and gneisses of the Penokee and Marquette districts of Michigan 

 and Wisconsin and the Black Hills of South Dakota, produced by the 



