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places to 5,000 or 6,000 feet, and the total area covered by 

 these trap rocks is about 200,000 square miles. The minerals 

 found in trap rocks are Quartz, Chalcedony, Agate, Jasper, 

 Limestone, and Zeolites. Soils formed from decomposition 

 of trap rocks are naturally very fertile, being rich in silica*, 

 alumina, iron, lime, magnesia, potash, phosphates and soda. 

 The celebrated Black cotton soil or Regur of Southern and 

 Central India was formed out of trap rocks probably in 

 shallow fresh water lakes. The Volcanic eruptions which have 

 given rise to the lava and tuff formations of Southern India, 

 took place in the cretacious period and the absence of vol- 

 canic craters in these regions, shows, that the eruption? 

 were fissure-eruptions giving rise to plateaux instead of hills 

 of Basalt. 



2nd. The Upper Magma resting on the Lower Magma 

 is lighter and is mainly composed of Silica. This stratum is 

 therefore called Siliceous or Acidic Magma. It is mainly 

 Plutonic, i. e.) solidified by slow cooling under pressure, and 

 occurring, in consequence, in the form of coarse grained 

 crystals compacted together in the form of granite. Acidic 

 rocks occasionally occur as Volcanic rocks, e.g. Trachyte, 

 Obsidian [and Pumice. The presence of Plutonic granite or 

 Volcanic Trachyte, Obsidian and Pumice, is an indication of 

 the earliest geological formation. It usually intrudes into 

 gneiss and it is sometimes difficult to distinguish between 

 intruded dykes or veins of granite and metamorphic schists 

 owing to gneissose structure of some of these granite veins. 

 Veins and dykes of granite occur throughout the vast meta- 

 morphic or gneissose rocks of India, all along the Himalayas, 

 in the Arravali hills, and mainly in the Deccan. Granite 

 consists of Quartz, Mica and Felspar, in varying proportions. 

 Quartz and Mica are not of much value as fertilizers, but 

 Felspar is, Soils formed out of granite are therefore less 

 fertile than those formed out of basalt. 



The Acidic rocks contain 60 to 75 /. f SiOa, the basic 

 C 



