Meaning of The Phrase "flinty bones"
I couldn't find the meaning of the said phrase on the web anywhere. Does it refer to human bones, animal bones or some other objects?
Thanks

"If the land had been any good the Hamiltons would have been rich people. But the acres were harsh and dry. There were no springs, and the crust of topsoil was so thin that the flinty bones stuck through. Even the sagebrush struggled to exist, and the oaks were dwarfed from lack of moisture. Even in reasonably good years there was so little feed that the cattle kept thin running about looking for enough to eat."
-East of Eden

Question

Meaning of The Phrase "flinty bones"
I couldn't find the meaning of the said phrase on the web anywhere. Does it refer to human bones, animal bones or some other objects? 
Thanks

"If the land had been any good the Hamiltons would have been rich people. But the acres were harsh and dry. There were no springs, and the crust of topsoil was so thin that the flinty bones stuck through. Even the sagebrush struggled to exist, and the oaks were dwarfed from lack of moisture. Even in reasonably good years there was so little feed that the cattle kept thin running about looking for enough to eat."
-East of Eden

Answer

Flinty bones means they're so old that they're flaking away. They're probably animal bones unless something earlier in the text indicates that there was some kind of human graveyard there.

Answer

'Flinty' typically refers to the nature of something being hard and unyielding like the stone.

Answer

Here is another excerpt that precedes the one I gave above. I am not sure if this is relevant. 
"The valley land was deep and rich, but the foothills wore only a skin of topsoil no deeper than the grass roots; and the farther up the hills you went, the thinner grew the soil, with flints sticking through, until at the brush line it was a kind of dry flinty gravel that reflected the hot sun blindingly."