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.
What was used in the mid-19th century to explain the presence of "folded" mountain chains? 
theory of geosynclines
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.
In the mid-nineteenth century the now defunct theory of geosynclines was used to explain the presence of "folded" mountain chains but by the mid-twentieth century the theory of plate tectonics became widely accepted.
What was used in the mid-nineteenth century to explain the presence of "folded" mountain chains? 