The World's Foundations; or, Geology for Beginners · Ch. V · Unit 1: Part I: How to read the record
5. What rocks are made of
Goal: Your child can name the three groups of water-made rock (flint, clay and lime rocks), test rocks for lime with vinegar, and do a simple scratch test for hardness.
You'll need
- The book, open to this chapter
- White vinegar and a dropper or spoon
- A sea-shell or piece of eggshell, and if you have them a piece of limestone, marble or natural chalk
- A few other rocks, including sand or a quartz pebble if you have one
- For the scratch test: a coin and a steel nail
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter V explains that about half the earth's crust is oxygen and a quarter silicon. It describes flint (quartz) rocks, clay rocks and lime rocks, how chalk is built from tiny shells, how coral makes limestone, the tiny flint-shelled diatoms, and how coal comes from buried forests.
The lesson, step by step
A question to start
SayThe air we breathe has a gas called oxygen in it. Would you believe that about half of solid rock is made of oxygen too?
That is the surprise this chapter opens with.
Read the chapter together
Read Chapter V (pages 42 to 52). Two numbers to stop on:
Yet one cubic inch of chalk is calculated to hold the remains of more than one million rhizopods.
Show a cubic inch with your fingers. Then read the coal passage:
Coal is a vegetable substance. The wide coal-fields of Britain and other lands are the fossil remains of vast forests.
Three families of rock
Flint rocks are made mostly of quartz (silicon joined to oxygen): flint, sandstone, beach sand, agate and amethyst. Clay rocks are made mostly of alumina. Lime rocks (limestone and chalk) are made of carbonate of lime, very often from shells, coral and tiny sea creatures.
The fizz test
Put a few drops of vinegar on the shell or eggshell. Watch closely: tiny bubbles mean lime. Try the limestone, marble or chalk if you have them, then a quartz pebble or sand.
Lime rocks fizz; flint rocks do not. Record each result in the rock journal. (Keep vinegar away from eyes, and rinse the rocks afterward.)
The scratch test
Hardness is another clue. Try to scratch each rock: first with a fingernail, then a coin, then the steel nail. Lime rocks are soft enough to scratch with the nail. Quartz and flint are so hard they scratch steel instead.
Safety: lay the rock flat on a table, hold it by the edges, and scratch away from your body, with an adult nearby. Rub the mark with a wet finger to check it is a real scratch, not a streak of metal.
Tiny builders
Talk about the book's tiny creatures: rhizopods with shells of lime (the chalk-makers), coral polyps (limestone-makers) and diatoms with shields of flint. Giberne says the white cliffs of England were built up from their shells on the sea floor and then lifted up.
Tell it back
Ask: What are the three families of water-made rock? How did the fizz test tell them apart? Where did coal come from, according to the book?
What we know now
Giberne's numbers hold up well: today's figures say oxygen is about 46 percent and silicon about 28 percent of the crust by weight. Her rhizopods (foraminifera) are now counted as single-celled living things rather than animals, and diatoms as tiny algae. Coal is still understood as the remains of ancient swamp forests.
Afterwards
Test every rock in your collection with vinegar and the scratch test, and label each "fizzes / doesn't fizz" and "nail scratches it / it scratches the nail".
Tip
Modern sidewalk chalk is often made of a different mineral and may not fizz. A sea-shell or eggshell always works.
Answers to the book's problems
Flint (silicious) rocks, clay (argillaceous) rocks, and lime (calcareous) rocks. Lime rocks fizz with vinegar; flint rocks do not. Coal, says the book, is the remains of ancient forests that grew in bogs and swamps, were buried under sand and mud, and slowly hardened.
Worked out for this site from the scan. If a number in your copy differs, trust the book.