Why can the geologic record be analyzed by looking at different rock layers?

The middle east links which two continents? ●asia and europe●europ and africa●africa and asia●africa and australia

This is a 2017 topographic map of puerto rico in the united states, and i'm sorry that people there have suffered such a disaster. i want to ask, what do the yellow dots and areas represent on this graph? and the blue area and the little blue circle, and the little blue sign that looks like a signal tower, what do they stand for?

All of the following occurred due to the invention of the railroad except