Observation of a "front" of regional metamorphism
Drawing his inspiration from the theories on metamorphism by reaction in the solid state, and from some observations, Rene Perrin in his article 'Perrin, Rene, Le metamorphisms generateur de plissement, Annales des Hines, Paris, October 1935.' "Metamorphism, the generator of folding" stated in 1935: 1) that some "sudden arrest" of regional metamorphism way occur and that some formations may present a barrier to its progress. It seemed to him that the Trias effectively played that role in some parts of the Alps (Perrin, Rene, p. 14 and 15). 2) p. 33 "A sedimentary formation overlying a folded crystalline rock in disconformity with the schistosity of the latter, does not prove that this rock (the crystalline) was formed and folded prior to the deposition of the sediment." Finally, he advised one (p. 32), before drawing any definite conclusion from the absence of contact metamorphism, to observe very closely the modifications either of the crystalline rock, or of the sedimentary formation, or of both in the vicinity of the contact, any modification being an indication of metamorphic action. Because these concepts seemed at first extremely bold, the authors made a thorough study of some specific cases.