Earth Study 



835 



5. Examine some sand with a lens. What mineral do you find present 

 in it in the greatest quantity? 



6. Write the story of the Cleopatra's Needle in Central Park, New 

 York City. 



CALCITE, MARBLE AND LIMESTONE 



Teacher's Story 



Calc spar, or calcium carbonate, is a mineral and is the material of which 

 marble, limestone and chalk are made. The faces of the calcite crystal are 

 always arranged in groups of three or multiples of three — a three-sided 

 pyramid or two pyramids joined base to base. The pyramids may be 

 obtuse or acute. When acute and formed of three pairs of faces, the 

 crystals are called dog-tooth spar. The crystals appear in a great variety 

 of forms, but they all have the common quality of splitting readily in three 

 directions, the fragments resembling cubes 

 which are oblique instead of rectangular. 

 When these cleaved, or split pieces, are trans- 

 parent, they are called Iceland spar. When an 

 object is viewed through Iceland spar at least 

 one-quarterinchthick, it appears double. The 

 calcite crystal is often transparent with a clight 

 yellowish tinge, but it also shows other colors; 

 and it has a slightly cloudy or slightly pearly or 

 almost glassy luster like feldspar. It is easily 

 scratched with a knife and will not scratch 

 glass. If a drop of strong vinegar or weak 

 hydrochloric acid falls upon it, it will efliervesce. 

 Limestone — so called because it is burned to 

 make quicklime — was formed on the bottoms 

 of oceans; its substance came chiefly from the 

 skeletons of corals and the shells of other sea 

 creatures, since sea-shells and coral stems are 



pure calcium carbonate in composition. In the water, the shells and 

 corals were broken down, and then deposited in layers on the bottom of the 

 sea. So wherever we find limestone, we know that there was once the 

 bottom of a great sea. Such layers of limestone are now being deposited off 

 the shores of Florida, where corals grow in great abundance. Limestone is 

 used extensively for building purposes, and in most climates is very durable. 

 The great pyramids of Egypt are of limestone. It is not a good material 

 for making roads, since it is so soft that it wears out readily, making a fine 

 easily-blown dust. It is slowly dissolved in water, especially if the water be 

 acid; thus, in limestone regions, there are caves where the water has 

 dissolved out the rock; and attached to their roofs and piled upon their 

 floors may be large icicle-shaped stalactites and stalagmites, which were 

 made by the lime-bearing water dripping down and evaporating, leaving its 

 burden in crystals behind it. When the roof of a cave falls in, the cavity 

 thus made is called a sink hole and is often dangerous. The famous Natural 

 Bridge in Virginia is all that is left of what was once the roof of such a 

 cavern. The water in limestone regions is always hard, because of the lime 

 which it holds in solution; and in such regions the streams usually have no 



Forms of calcite crystals. 



