KINDS OF ROCKS. 435 



3. The Mica and Soda-lime-Feldspar series. These grani- 

 toid rocks are equally alkali-yielding with those of the true 

 granite group, but the alkali is mainly soda. The nephelite 

 (elaeolite) rocks not hornblendic are here included, although 

 they contain in general some microline or orthoclase. 



4. The Hornblende and Potash-Feldspar series, or the Sye° 

 nyte group. In this series, the mica of the granite series is 

 replaced by the non-alkaline mineral, hornblende. Transi- 

 tions between the granite and syenyte rocks are common — a 

 bed that is true mica schist often becoming hornblendic ; 

 the same specimen may have mica and hornblende ciwstals 

 together, or parallel mica and hornblende layers, and then 

 not far beyond the schist may be a purely hornblende rock ; 

 and so there are similar transitions in other parts of the 

 two series. This transition in a stratum of mica schist, a 

 metamorphic rock, indicates merely that the mud-bed or 

 sedimentary stratum, out of which the mica schist was 

 made, had a diminished proportion of alkali in some parts, 

 and, in still others, a complete absence of alkali — which is 

 just such a variation as might be looked for in oceanic sedi- 

 ments, as they spread over a wide region. The hornblende 

 may be replaced by epidote, another iron-bearing mineral. 



5. The Hornblende and Soda-lime-Feldspar series. The 

 soda-lime-feldspars, in this series, may be either of the 

 triclinic species, from albite to anorthite, 



6. The Pyroxene and Soda-lime-Feldspar series. The soda- 

 lime-feldspars are the same as in the })receding. Quartz 

 is very rarely present, except in traces. Potash replaces 

 soda in amphigenyte. 



7. Pyroxene, Garnet, Epidote and Chrysolite rocks, con- 

 taining little or no Feldspar. 



8. Hydrous Magnesian and Aluminous rocks, 

 &. Iron Ore rocks 



1. SILICEOUS ROCKS. 



1. Quartzyte, Granular Quartz. — A siliceous sandstone, 

 usually very firm, occurring in regions of metamorphic 

 rocks. It does not differ essentially from the harder sili- 

 ceous sandstones of other regions. Conglomerate beds are 

 sometimes included. 



Varieties. — a. 3fassive. b. Sc7dstose. c. Calcareous; sometimes 

 contains disseminated calcite which, where the rock is exposed to 

 weathering, is removed and leaves the rock loose in texture, or cellu- 



