836 Handbook of Nature-Study 



silt, but have clean bottoms; moreover, the springs are likely to become 

 contaminated because the water has run through long caves instead of 

 filtering through sand. 



Chalk is similar in origin to limestone; it is made up of the shells of 

 minute sea creatures, so small that we can only see them with the aid of a 

 microscope. Try and think how many years it must have required for the 

 shells of such tiny beings to build up the beds which make the great chalk 

 chfEs of England ! 



Marble is formed inside of the earth from limestone, under the influence 

 of heat and pressure ; it differs from limestone chiefly in that the grains are 

 of crystalline structure, and are larger; it is usually white or gray in color, 

 and sometimes is found in differing colors. At Cadiz in California, marble 

 is found showing twenty or more quite different colors. The most famous 

 marbles are the Carrara of Italy, the Parian from the Island of Paros, and 

 the Pentelican from the mountain of that name near Athens. The reason 

 why these marbles are so famous is that in ancient times sculptors carved 

 beautiful statues from them, and the architects used them for building 

 magnificent temples. The principal marble deposits in the United States 

 are in Vermont, Georgia, Tennessee and California. Marble deteriorates 

 when it is exposed to air which is filled with smoke and gases. It is also 

 used to make lime. When either marble or limestone is heated very hot, it 

 separates into two parts, one of which is lime, and the other carbonic acid 

 gas — the same that is used for charging soda-water fountains. 



' LESSON CCXVIII 



Calcite, Marble and Limestone 



Leading thought — Calcite or calc spar is formed more than half of lime. 

 The best known forms of its crystals are cubelike, but instead of having 

 twelve right-angled edges, the sides are lozenge-shaped, and are set together 

 with six obtuse angles and six acute. Dog-tooth spar is one form of calcite 

 crystal. Limestone is a solid form of calcite. Marble is granular lime- 

 stone which shows the broken crystals of calcite. Chalk is very fine, 

 pulverized calcite. 



Method — Specimens of dog-tooth spar, limestone, marble, shells of 

 oysters or other sea creatures and coral should be provided for this lesson; 

 also a bottle of dilute hydrochloric acid, and a piece of glass tubing about 

 six inches long with which to drop the acid on the stones. Some strong 

 vinegar will do instead of the acid. 



Observations — i. What is the form of the calcite crystal ? What is the 

 luster of the crystal ? Is it the same as the inside of sea-shells ? Will cakite 

 scratch glass ? Can you scratch it with a knife ? What happens to calcite 

 if you put a drop of weak hydrochloric acid upon it ? 



2. Is marble made up of crystals? Examine it with a lens to see. 

 What is its color ? Have you seen marble of other colors than white? Do 

 you know the reason why marble is sometimes clouded and streaked? 



3 . Put a drop of weak hydrochloric acid on the marble. What happens ? 



4. What are the uses of marble ? What have you ever seen made from 

 marble? Why is it used for sculpture? What famous statues have you 

 seen which were made of marble ? Name some of the famous ancient marble 

 buildings. 



