In short, they were just natural buttresses, without which the stem could not have supported its heavy and umbrageous top.

We found these chestnuts to be very numerous.

They grew chiefly on the banks of the stream, and were of all sizes.
While we were examining a small tree of this kind, Jack chipped a piece off a buttress with his axe, and found the wood to be firm and easily cut.

He then struck the axe into it with all his force, and very soon split it off close to the tree, first, however, having cut it across transversely above and below.

By this means he satisfied himself that we could now obtain short planks, as it were all ready sawn, of any size and thickness that we desired; which was a very great discovery indeed, perhaps the most important we had yet made.
We now wended our way back to the coast, intending to encamp near the beach, as we found that the mosquitoes were troublesome in the forest.