Some American Eruptive Granites

Two hypotheses, diametrically opposed, have been entertained by geologists in explaining the origin of large granitic masses. According to the one a granite is the last stage in the metamorphic alteration of mechanical sediments. According to the other a granitic mass is the product of the gradual cooling of an acidic molten magma under pressure. In the first case it is claimed that all the gradations have been traced in the same mass from undoubted elastics through slaty, schistose and gneissic phases to the truly granitic types; so that one end of an originally sedimentary deposit may be now unaltered, while the other end is a true holo-crystalline rock. On the other hand unquestionable eruptive granites are known to pass into gneiss and even into the schistose stages, through the agency of enormous compression. Regarding the facts deduced in the support of the first assumption and without referring to any specific instances, it seems quite probable that in the majority of cases bearing directly on this point sufficient discrimination has not been exercised along the lines separating the semi from the holo-crystalline areas