During the time of the dinosaurs, magma rose to the surface, as Pangea started to break up, forming the Atlantic Ocean, http://www.nhgeology.org/jurassic.htm. The rising magma formed volcanoes in places in New Hampshire, including the great ring dike that is the Osispee Mountains. Basalt forms at the surface. Granite forms underneath other rock layers taking longer to cool so crystals can form. Granite at the surface means other rock like basalt which cooled quickly has eroded.