13(5 



sometimes penetrating them in the form of dikes and veins. It 

 is usually composed of three simple minerals, felspar, quartz, 

 and mica, and derives its name from having a coarse granular 

 structure ; granum, Latin for grain. 



Graywacke. Gramvacke, a German name, generally adopted by 

 geologists for the lowest members of the secondary strata. 

 The rock is very often of a gray colour, hence the name, 

 grau, being German for gray, and wacke being a provincial 

 miner's term. 



Greensand. Beds of sand, sandstone, limestone, belonging to the 

 Cretaceous period. The name is given to these beds because 

 they often, but not always, contain an abundance of green 

 earth or chlorite scattered through the substance of the sand- 

 stone, limestone, &c. 



Greenstone, a variety of trap, composed of hornblende and felspar. 



Grit, a provincial name for a coarse-grained sandstone. 



Gypsum, a mineral composed of lime and sulphuric acid, hence 

 called also sulphate of lime. Plaster and stucco are obtained 

 by exposing gypsum to a strong heat. It is found so abun- 

 dantly near Paris, that Paris plaster is a common term in this 

 country for the white powder of which casts are made. The 

 term is used by Pliny for a stone used for the same purposes by 

 the ancients. The derivation is unknown. 



Hornblende, a simple mineral of a dark green or black colour, which 

 enters largely into the composition of several varieties of the 

 trap rocks. 



llornstone. A silicious mineral substance sometimes approaching 

 nearly to flint, or common quartz. It has a conchoidal frac- 

 ture, and is infusible, which distinguishes it from compact 

 felspar. 



Jura Limestone. The limestones belonging to the oolitic group, 

 constitute the chief part of the mountains of the Jura, between 

 France and Switzerland, and hence the geologists of the Con- 

 tinent have given the name to the group. 



Laminae. Latin for plates ; used in geology, for the smaller layers 



of which a stratum is frequently composed. 

 Lava. The stone which flows in a melted state from a volcano. 



