Context: Important geological concepts were established as naturalists began studying the rock formations of the Alps in the 18th century. In the mid-19th century the now defunct theory of geosynclines was used to explain the presence of "folded" mountain chains but by the mid-20th century the theory of plate tectonics became widely accepted.

Question: Who began studying the rock formations of the Alps
Answer: naturalists

Question: When did the studying of rock formations begin in the Alps?
Answer: the 18th century

Question: What was used in the mid-19th century to explain the presence of "folded" mountain chains? 
Answer: theory of geosynclines

Question: What theory was more widely accepted by the mid-20th century? 
Answer: the theory of plate tectonics