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rocks less than 50 / . The acidic rocks are light, and more 

 infusible, while the basic rocks are very heavy and of fine 

 texture and they are not so infusible. The principal acidic 

 rocks are : Granite^ Pel site, Obsidian, Pumice, Syenite, 

 Trachyte, and Porphyrite. The principal basic rocks are 

 Basalt, Dolerite, Diorite, and Gabbro ( containing Diallage 

 and Labradorite.) 



3rd. The Azoic or metamorphic rocks. These consist 

 of gneiss, mica-schist and clay-slate, formed by the joint 

 action of sedimentation in water and compaction by heat. 

 They are called Azoic because no trace of life has been dis- 

 covered in them. There are three distinct systems of Azoic 

 rocks which from above downwards may be called the 

 Vindhyan, the Sub metamorphic and the Me-tamorphic. The 

 Vindhyan system consists of sandstones, limestones, shale 

 and iron pyrites, and the dendritic markings of earthy 

 manganese oxide which may be easily mistaken for fossil 

 plants are characteristic of this system. The Sub-meta- 

 morphic system consists of Quartzite, sandstone, slate, shale 

 and limestone of more igneous or crystalline appearance. 

 The older and still more crystalline rock which abounds in 

 Southern India is called gneiss. More than half the Penin- 

 sular area is on gneiss. From Cape Comorin to Colgong 

 on the Ganges, a distance of 1400 miles with a^mean 

 width of 350 miles, or an area of nearly 500,000 square 

 miles, is composed of gneiss or soils formed mainly out 

 of gneiss. Patches of newer strata occur here and there 

 on the gneiss. The Bundelkhand gneiss is the oldest of 

 all. Gneiss also occurs in the Himalayas, in the Chutia 

 Nagpur Division and in Assam. It is composed of Quartz, 

 Felspar, Hornblende, Chlorite, and Mica, all or only two of 

 which minerals may be present. Lead, silver, garnet, corun- 

 dum, and diamond are occasionally found in Azoic rocks. 

 The lead-ore or Galena of Bhagalpur is argentiferous. Lead- 

 ores occur in Chutia Nagpur also. Diamond occurs chiefly 



