COMMON ROCKS 



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Figure 1 — Massive Granite Stone Mountain, De Kalb County, Ga. 

 (Walson, U. S. Geol. Sur.) 



forms are known as batholiths. When the molten mass flows 

 cut upon the surface of the earth it is called an eruptive or 

 volcanic rock. Such rocks are also known as lavas. They may 

 occur either as great flows covering many thousands of square 

 miles or the molten material may pile up forming mountains which 

 are commonly known as volcanoes when lava is still erupted. 

 Since the molten mass which flows out on the surface cools quick- 

 ly, the various minerals that would have formed had the mass 

 been an intrusive one, do not have time to grow so such rocks 

 are generally glassy or the crystals are so very fine that they 

 cannot be seen with the naked eye or even with a good lenses. 



Igneous rocks are distinguished from other rocks by their 

 massive and usually grained character. (See figures 1 and 4.) 

 There are no bedding planes such as are found in sedimentary 

 rocks and they are not banded like the metamorphic rocks. Since 

 each rock is composed of varying proportions of the rock-form- 

 ing minerals and since the rate of cooling is not the same for all 

 rocks it is to be expected that the sizes of the minerals and their 

 relation to each other will vary widely. This feature is known 

 as the texture of rocks. Thus, we have coarse-grained textures, 

 fine-grained textures, glassy textures, porphyritic textures, etc. 

 We have a porphyritic texture when some of the crystals of the 

 rock are larger than the remainder. (See figure 5). Such a 



