The Primary strata, at the bottom of the pile, were tilted and lacked fossils. Next, were the Secondary strata, which were tilted but had fossils in them. On top were the Tertiary strata, which were horizontal and also contained fossils. He added a fourth class, Quaternary, for the sands and gravels that covered the bedrock in the nearby Po River Valley.
Today, scientists name the Primary and Secondary strata and the corresponding eras on the geologic time scale Paleozoic (about 540 to 250 million years ago) and Mesozoic (about 250 to 65 million years ago). Arduino's Tertiary and Quaternary divisions still stand . The latter two are the main sub-divisions of the Cenozoic, spanning from the end of the Cretaceous (the last period in the Mesozoic) to the present .
Keywords: geology, stratigraphy, geologic time scale, natural history, terminology
References and more to explore
 Giovanni Arduino [www.strangescience.net/arduino.htm].
 Alan Cutler: The Seashell on the Mountaintop. Dutton (Penguin Group), New York, 2003; page 196.
 Introduction to the Cenozoic - 65 Million Years to the Present [www.ucmp.berkeley.edu/cenozoic/cenozoic.html].