Excerpted from Where I Lived, and What I Lived For by Henry David Thoreau:
This small lake was of most value as a neighbor in the intervals of a gentle rain-storm in August, when, both air and water being perfectly still, but the sky overcast, mid-afternoon had all the serenity of evening, and the wood thrush sang around, and was heard from shore to shore. A lake like this is never smoother than at such a time; and the clear portion of the air above it being, shallow and darkened by clouds, the water, full of light and reflections, becomes a lower heaven itself so much the more important. From a hill-top near by, where the wood had been recently cut off, there was a pleasing vista southward across the pond, through a wide indentation in the hills which form the shore there, where their opposite sides sloping toward each other suggested a stream flowing out in that direction through a wooded valley, but stream there was none. That way I looked between and over the near green hills to some distant and higher ones in the horizon, tinged with blue. Indeed, by standing on tiptoe I could catch a glimpse of some of the peaks of the still bluer and more distant mountain ranges in the northwest, those true-blue coins from heaven's own mint, and also of some portion of the village. But in other directions, even from this point, I could not see over or beyond the woods which surrounded me. It is well to have some water in your neighborhood, to give buoyancy to and float the earth. One value even of the smallest well is, that when you look into it you see that earth is not continent but insular. This is as important as that it keeps butter cool. When I looked across the pond from this peak toward the Sudbury meadows, which in time of flood I distinguished elevated perhaps by a mirage in their seething valley, like a coin in a basin, all the earth beyond the pond appeared like a thin crust insulated and floated even by this small sheet of intervening water, and I was reminded that this on which I dwelt was but dry land.
What does the bolded sentences mean? Is continent an adjective there? But I can't find proper definition of it as an adjective in dictionary. And finally, why it mentions butter and why a well can keep butter cool?