ROCKS. 67 



CHAPTER Yin. 



BOCKS. 



137. Connection of Mineralogy with Geology. In this 

 chapter we shall pass over ground that connects miner- 

 alogy with geology. We have looked at minerals in de- 

 tail and in their separate condition, merely alluding to 

 them occasionally as being masses, but now we are defi- 

 nitely to consider these masses preparatory to a view of 

 them as a whole, making up the crust of the earth. It 

 is such a view that constitutes Geology, the term being 

 derived from two Greek words, ge, earth, and logos, dis- 

 course. 



138. Definition of Rock. The word rock, in common 

 language, is applied only to such mineral aggregates of 

 considerable size as are solid ; but the geologist gives it 

 a wider meaning. He speaks of all such aggregates as 

 rocks, whether they are solid or made up of loose mate- 

 rial. Thus the clay slate, and the strata of clay mud, 

 which, by long-continued pressure, or this and heat to- 

 gether, might ultimately become clay slate, are both 

 equally regarded as rock. The same may be said of 

 sandstones and deposits of sand. The geologist, how- 

 ever, often uses the word rock in its ordinary sense, the 

 context showing in which sense he does use it. 



139. Elementary Substances in the Rocks. There are 

 between sixty and seventy elements, but only nine of 

 them enter to any great extent into the composition of 

 the rocks, viz., oxygen, silicon, aluminum, calcium, mag- 

 nesium, potassium, sodium, iron, and carbon. Of these 

 only one, oxygen, is a gas, but that is so abundant that 

 it is supposed to constitute nearly or quite one half of 

 all the ponderable matter in the globe. These elements, 



