The World's Foundations; or, Geology for Beginners · Ch. VI · Unit 1: Part I: How to read the record
6. Rock-layers
Goal: Your child can explain how fossils help geologists match rock layers in different countries, and can read the book's table of strata from oldest to newest.
You'll need
- The book, open to this chapter
- About 12 index cards or paper strips in four colors
- Colored pencils
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter VI warns that geology must be read with a "perhaps". It explains that rock layers always follow the same order, that fossils change as you go down through the layers, and that the same fossils in England and America show rocks of the same age. It ends with the book's Table of Strata.
The lesson, step by step
A question to start
SayA cliff in England is made of sandstone, and a cliff in America is made of limestone. How could anyone tell whether they were made at the same time?
Keep the question in mind while reading.
Read the chapter together
Read Chapter VI (pages 53 to 60). Two sentences to stop on:
It is very needful, therefore, to hold the conclusions of geologists with a light grasp, guarding each with a " perhaps " or a " may-be."
Descend through layer after layer till we come to rocks built in earliest ages, and not one fossil shall we find precisely the same as one animal living now.
Fossils tell the time
In plain words: the deeper the layer, the stranger the fossils. Certain fossils are found only in certain layers, all over the world. So if two rocks hold the very same kinds of fossils, they were probably made in the same age, even if the rocks themselves are different.
Match two cliffs
Make two "cliffs" of four cards each. On each card in the first cliff write a rock name (sand, clay, lime, coal) and draw a simple fossil (a snail, a fish, a fern, a three-lobed bug). Make the second cliff with different rock names but the same four fossils in the same order.
Shuffle each pile, then ask your child to stack both cliffs and line up layers of the same age by their fossils.
Read the table
Turn to the Table of Strata at the end of the chapter. It has three great divisions:
PRIMARY, OR PALEOZOIC, OR ANCIENT-LIFE ROCKS.
SECONDARY, OR MESOZOIC, OR MIDDLE-LIFE ROCKS.
TERTIARY, OR CAINOZOIC, OR NEW-LIFE ROCKS.
Read the names under each from the bottom (oldest) up. The book says to look back at this list often in Part II.
The order never flips
Read:
The Devonian may be absent, but if present, you will never find it under the Silurian, or above the Carboniferous.
Ask: Could a layer be missing? (Yes.) Could the order be swapped? (Not in the usual way of things.)
A memory hook
Have your child make up a silly sentence to remember the book's order: Cambrian, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous. Write it in the rock journal.
What we know now
Fossils are still used exactly this way to match rocks. Today's list has one more period, the Ordovician, between the Cambrian and the Silurian (it was named in 1879, and the book does not use it). Giberne's Laurentian rocks are now part of the Precambrian, which covers most of the earth's history. Geologists can now also put dates on the table: the Cambrian began about 539 million years ago.
Afterwards
Say the order of the ages each day this week until it comes easily. Copy the Table of Strata into the rock journal as a tall column, oldest at the bottom.
Answers to the book's problems
Two rocks with the same kinds of fossils, different from the layers above and below, probably belong to the same age. The oldest layers lie lowest; the order of the layers is the same everywhere, though some layers may be missing.
Worked out for this site from the scan. If a number in your copy differs, trust the book.