The World's Foundations; or, Geology for Beginners · Ch. XXII, part 2 · Unit 3: Part II: Reptiles, chalk, mammals, ice and people
25. Reading the past by the present, and the table of ages
Goal: Your child can explain the "uniformity" idea and the author's caution about it, and can read the book's Table of the Ages of Geology with the meanings of the names.
You'll need
- The book, open to this chapter
- A roll of toilet paper (about 50 squares) or a long strip of paper
- A marker and small sticky notes
- A globe, atlas or free online map
What's in the book
The second part of Chapter XXII (pages 208 to 216) introduces Part III: studying the forces at work today to understand the past. Giberne cautions against assuming those forces always worked at today's speed (the "uniformity theory"). It ends with a fuller Table of the Ages of Geology, explaining where names like Cambrian, Devonian, Permian and Jurassic come from.
The lesson, step by step
A question to start
SayIf a puddle dries up in one day, does that mean every puddle that ever existed took exactly one day to dry?
This is the heart of today's caution.
Read together
Read pages 208 to 216. Two sentences to compare:
We may fairly and rightly examine the working of the different forces as seen in the present day, and may learn much from that working about their action in past ages.
and her warning that this reasoning can be "pushed too far", if we assume the past always went at today's speed.
Both sides in plain words
Uniformity: the same forces (rain, rivers, waves, volcanoes) shaped the earth in the past as now, and at about the same speed. Giberne agrees the forces are the same, but says we cannot know the speed was always the same: floods, eruptions, a hotter early earth may have changed things.
Where the names come from
Look at the Table of the Ages of Geology. Find the places on a map: Wales (Cambrian, Silurian), Devonshire (Devonian), Perm in Russia (Permian), the Jura Mountains (Jurassic). For example:
DEVONIAN AGE ; also known in part as Old Red Sandstone. Called Devonian because largely visible in Devonshire.
A toilet-paper timeline
Unroll 45 squares along a hallway. Each square stands for 100 million years, so the whole roll is the earth's 4.5 billion years. Put the "today" end at the door. Mark with sticky notes:
Trilobites and the Cambrian begin: about 5 and a half squares from the end.
Ancient-Life Period ends: about 2 and a half squares from the end.
Great reptiles end: two-thirds of one square from the end.
The Ice Ages: about the last tenth of an inch.
People: thinner than a pencil line at the very end.
Tell it back
Ask your child to explain the "uniformity" idea and Giberne's caution about it. Then: walk along the timeline and tell what the book says happened in each age.
What we know now
Geologists today use both of the ideas Giberne weighs. The forces we can watch now explain most of the rocks, and rare, sudden events (huge eruptions, great floods, an asteroid strike) also left their marks. The earth was once hot and molten, but it was never a sun shining by its own light. The modern table adds the Ordovician, and puts dates on it: the Ancient-Life Period ran from about 539 to 252 million years ago, the Middle-Life from 252 to 66, and the New-Life from 66 million years ago to today.
Afterwards
Leave the timeline up for the week and add a sticky note for each lesson of Part II you remember.
Answers to the book's problems
Uniformity is the idea that the forces shaping the earth have always worked the same way and at the same rate as today. Giberne agrees we learn much from the present, but warns we cannot be sure the rate was always the same, since conditions in the past may have been very different.
Worked out for this site from the scan. If a number in your copy differs, trust the book.