John Smith Flett

“ MYLONITE (Gr. μυλών, a mill) , in petrology, a rock which has been crushed and ground down by earth movement and at the same time rendered compact by pressure. Mylonites are fine-grained, sometimes even flinty, in appearance, and often banded in parallel fashion with stripes of varying composition. The great majority are quartzose rocks, such as quartzite and quartz-schist; but in almost any type of rock mylonitic structure may be developed. ”

“ Thrust planes and great reversed faults are often bounded by rocks which have all been crushed to fine slabby mylonites, that split readily along planes parallel to the direction in which movement has taken place. ”

“ Crushing and movement on so extensive a scale are to be expected principally in regions consisting of rocks greatly folded and compressed. Hence mylonites are commonest in Archean regions, but may be found also in Carboniferous and later rocks where the necessary conditions have prevailed. Within a short space it is often possible to trace rocks from a normal to a highly mylonized condition, and to follow by means of the microscope all the stages of the process. ”