But that isn't all of this business--this partnership--between the volcanoes and the winds. Did anybody ever tell you how the volcanoes help the winds to help the plants to get their breath? It's curious. And more than that, it's so important--this part of the work--that if it weren't carried on in just the way it is, we'd all of us--all the living world, plants and animals--soon mingle our dust with that of the early settlers we read about in the last chapter. In other words, all the _plant_ world would die for lack of fresh air and all the _animal_ world would die for lack of fresh vegetables. So they say!
According to that fine system--the breath exchange between the people of the plant and animal kingdoms--the plants breathe in the carbon gas that the animals breathe out; you remember about that. But the amount of carbon gas in the air is never very large, and if there were no other supply to draw on except the breath of animals and the release of this same gas when the plants themselves decay, we'd very soon run out.
Now this needed additional supply comes from the volcanoes. Every time a volcano goes off--and they're always going off somewhere along the world's great firing-line--it throws out great quantities of this gas, and this also the winds distribute widely and mix through the atmosphere.
And another thing: This carbon in the air helps crumble up the rocks already made, and it enters into the manufacture of the limestone in the rock mills of the sea. This limestone will make just as rich soil for the farmers of the future as the limestones of other ages have made for the famous Blue-Grass region of Kentucky, for example.
All of which only goes to show how first unpleasant impressions about people and things are often wrong. A "dusty March day," you see, isn't just a dusty March day. It's quite an affair!
But wind is not alone a carrier for other dust-makers; it has dust mills of its own. The greatest of these mills are away off among the mountains and in desert lands, but after making it in these distant factories the winds carry much of this fresh new soil material to lands of orchard and pasture and growing grain.
Not long ago two of the professors at the University of Wisconsin found a good illustration of what an immense amount of soil is distributed in this way, and what long distances it travels. Among the weather freaks of a March day was a fall of colored snow that, it was found, covered an area of 100,000 square miles, probably more. The color on the snow was made by dust blown clear from the dry plains of the Southwestern States, a thousand miles away. The whole of this dust amounted to at least a million tons; and may even have amounted to hundreds of millions of tons, so the professors think.