strata

The older strata gradually disintegrated.

These strata are the product of those billions and billions of shells.

It is not surprising, therefore, that the highest ratio of oil and gas pools to volume of sediment is found in rock no older than 2.5 million years - young enough so that little of the petroleum has leaked away - and that nearly 60 percent of all oil and gas discovered so far has been found in strata that formed in the last 65 million years .

When archaeologists are digging at a site, they make very careful notes about which stratum (strata), which layer of earth they find things in.

He later worked on similar jobs across the length and breadth of England, all the while studying the newly revealed strata and collecting all the fossils he could find.

In 1815 he published the first modern geological map, “A Map of the Strata of England and Wales with a Part of Scotland,” a map so meticulously researched that it can still be used today.

Ever since people had begun to catalog the strata in particular outcrops, there had been the hope that these could somehow be used to calculate geological time.

But as more and more accumulations of strata were cataloged in more and more places, it became clear that the sequences of rocks sometimes differed from region to region and that no rock type was ever going to become a reliable time marker throughout the world.

As he collected fossils from strata throughout England, Smith began to see that the fossils told a different story from the rocks.

Particularly in the younger strata, the rocks were often so similar that he had trouble distinguishing the strata, but he never had trouble telling the fossils apart.

While rock between two consistent strata might in one place be shale and in another sandstone, the fossils in that shale or sandstone were always the same.

Some fossils endured through so many millions of years that they appear in many strata, but others occur only in a few strata, and a few species had their births and extinctions within one particular stratum.

Not only could Smith identify rock strata by the fossils they contained, he could also see a pattern emerging: certain fossils always appear in more ancient sediments,  while others begin to be seen as the strata become more recent.

By following the fossils, Smith was able to put all the strata of England's earth into relative temporal sequence.  

Limestone may be found in the Cambrian or—300 million years later—in the Jurassic strata, but a trilobite—the ubiquitous marine arthropod that had its birth in the Cambrian—will never be found in Jurassic strata, nor a dinosaur in the Cambrian.

Other conspiracies in the late eighteenth century as well as a brief revolt in 1817 reflected the influence of republican ideas over sections of the elite and even the lower strata of urban society.