8 Nature's 



If there is a molten center to the earth these 

 igneous rocks are undoubtedly the offspring 

 of this great internal furnace. The meta- 

 morphic rocks were primarily igneous and are 

 changed somewhat in their structure by the 

 lapse of time. For instance, marble is a meta- 

 morphic limestone. The difference between 

 common limestone and marble is in its molec- 

 ular structure the way in which its smallest 

 particles are put together. They are both 

 carbonates of lime. But the marble is made 

 up of little crystals and will take a polish, 

 while ordinary uncrystallized limestone will 

 not. The igneous rocks are chiefly granite; 

 and granite is formed of orthoclase-feldspar, 

 mica, and quartz. (The word " orthoclase " 

 means straight fracture, and the orthoclase- 

 feldspar has two lines of cleavage at right 

 angles to each other.) This is the ordinary 

 composition of granite, but there are a great 

 many variations, chiefly as to color and pro- 

 portions of the ingredients named. 



The igneous rocks, then, are the lowest of 

 all; then come the metamorphic rocks; and 

 as before stated, on top of metamorphic 

 rock begins the first evidence of life in its 

 lowest form. The Paleozoic (ancient life) or 

 Primary period is made up of a number of 

 subdivisions. The first and oldest division is 

 called the " Silurian " age, which is underlaid 

 by the metamorphic rocks and overlaid by the 



