The Laurentian rocks and metamorphism

Abstract

In the early days of geology, as geologists are aware, granite and the associated highly metamorphic rocks were called ‶Primary Rocks,″ as from their position they were supposed to be the oldest rocks. This misnomer, in the early days of geology, was not an unnatural one, as these early observers supposed that strata accumulated by a process very similar to building a wall, the upper stones having been placed last; they having yet to learn that younger rocks might be thrust up from below, while in many cases they carry up on them masses of much older rocks. Thus, in some cases, position may not be indicative of age, as part of such thrust-up younger rocks may be below older rocks. As geology advanced, more minute and extended examination gradually led to this supposition being abandoned, as it was found that the supposed ‶Primary Rocks,″ although lithologically similar, might differ greatly as to age; also that they were often younger than the rocks with which they were intimately connected, as the intrusive granites must be younger than the rocks through which they come up, while the associated metamorphism must have taken place subsequent to the original rocks having accumulated. The Laurentians of Canada and Scotland are known by ocular proofs to be older than the associated rocks of Cambrian age. These rocks, during the vicissitudes which they have undergone, have been subjected to more or less metamorphic action, and have therefore taken certain mineralogical or lithological characters—characters which, although very