Final answer:
To show that mountain ranges on opposite sides of the Atlantic formed as two parts of the same range and were once joined, you would look for similar rock types, matching geologic structures, and evidence of ancient mountain-building events.
Step-by-step explanation:
To show that mountain ranges on opposite sides of the Atlantic formed as two parts of the same range and were once joined, you would look for several pieces of geologic evidence. One piece of evidence would be similar rock types and formations on both sides of the Atlantic. For example, if you find the same types of sedimentary rocks and fossils in mountain ranges in Europe and North America, it would suggest that they were once connected.
Another piece of evidence would be matching geologic structures, such as fault lines or fold patterns, that extend from one side of the Atlantic to the other. These structures would indicate that the mountain ranges were once part of the same tectonic event. Lastly, you could look for the presence of ancient mountain-building events, such as the collision of tectonic plates, which could be recorded in the geologic record on both sides of the Atlantic.