VOLCANIC ROCKS. 29 



hornblende, in a glassy ground mass, with a little 

 mica and magnetic iron. The quartz hornblende 

 andesites correspond to syenites in chemical com- 

 position ; just as syenites correspond chemically 

 to some Cambrian slates. The hornblende ande- 

 ) sites, which are free from quartz, are closely re- 

 lated to the rocks named diorites. Andesites are 

 largely quarried on the Rhine, in the Siebenge- 

 birge, near the Apollinaris spring at Remagen. 

 Andesite abounds in black concretions rich in 

 hornblende, like those found in the granite of 

 Shap in Westmoreland. Phonolite is probably a 

 volcanic representative of a syenite which con- 

 tains the mineral nepheline. 



Basalt — This is the most familiar volcanic 

 rock. Its silica is 1 to 35 to 55 per cent. 



Oxide of iron, lime, and magnesia are more abun- 

 dant in it than in otln mic rocks. It con- 

 iefly of the minerals labrador-felspar, and 

 augite, or some similar sul . usually associ- 

 ated with a little magnetite and olivine. It is 

 dark in tint, grey-brown, blue-black, or greenish 

 black when freshly broken. Cooled slowly, it 

 gains a fine granular texture, and is known as 

 dolerite. 



In the most ancient basalts of Cambrian, 

 Silurian and Devonian ages the olivine and augite 

 have been partly decomposed, and converted into 

 a green mineral like chlorite. r I he ba>alt or 

 dolerite is then known as diabase. The less 

 altered dolerites, of carboniferous age, have been 

 termed melaphyres. Occasionally the felspar in 

 basalt may be replaced by allied minerals. In 

 Etna and Vesuvius leucite takes its place. There 

 is also a nepheline basalt. 



Olivine may take the place of felspar. That 



