Wherever you see a bit of blue, which is the mark for limestone, you may say, "There is a bit of old coral-reef rising up to the surface." But because I will not puzzle your little head with too many things at once, you shall look at one set of coral-reefs which are far newer than this bit of Dudley limestone, and which are the largest, I suppose, that ever were in this country; or, at least, there is more of them left than of any others.
Look first at Ireland.

You see that almost all the middle of Ireland is coloured blue.

It is one great sheet of old coral-reef and coral-mud, which is now called the carboniferous limestone.

You see red and purple patches rising out of it, like islands--and islands I suppose they were, of hard and ancient rock, standing up in the middle of the coral sea.
But look again, and you will see that along the west coast of Ireland, except in a very few places, like Galway Bay, the blue limestone does not come down to the sea; the shore is coloured purple and brown, and those colours mark the ancient rocks and high mountains of Mayo and Galway and Kerry, which stand as barriers to keep the raging surf of the Atlantic from bursting inland and beating away, as it surely would in course of time, the low flat limestone plain of the middle of Ireland.

But the same coral-reefs once stretched out far to the westward into the Atlantic Ocean; and you may see the proof upon that map.