Page:The Journal of geology (IA journalofgeology21894univers).pdf/25

only traces of their original forms, what may we expect of those of Paleozoic or Archean times?

On the other hand, the association in dissected volcanic regions of the effusive rocks with correspondingly abyssal types naturally suggests that volcanoes may have once surmounted many areas of coarsely granual ancient igneous rocks. As this, however, cannot be proved, only such regions are here considered as yield rocks of unmistakably surface origin.

Again, ancient volcanic rocks may have been subjected to metamorphosing processes severe enough to have destroyed most of their original characters. In such cases, patient study and a careful weighing of all evidence is necessary to decide their origin, and even that may not avail. Igneous rocks may be so altered as to be indistinguishable from metamorphosed sediments, but it many cases where this at first appears to be the fact, some decisive clue may be discovered.

In establishing the volcanic nature of rocks occurring in ancient and more or less crystalline terrains, attention must be given to several different sets of characters. The field relations must be carefully studied and the material collected on the spot and afterward studied in the laboratory. The criteria for deciding on their igneous and volcanic origin may be arranged as follows:

1. If the rocks are igneous, whether abyssal or surface, they will:
1. Conform in chemical composition to certain well established types;
2. Show an association of petrographical types which, both chemically and mineralogically, follow the laws of consanguinity.
2. If they are volcanic:
1. They may be found in the field to occur in distinct sheets, flows or necks;
2. They will have produced very little or no contact action in the adjoining rocks;
3. They may include irregular fragments of other rocks.