Page:The Journal of geology (IA journalofgeology11893univers).pdf/262

As has already been stated in an earlier paper,^[2] the writer purposes, as time and opportunity permit, to discuss the petrographical and stratigraphical relationships of the basic rocks that constitute such an important element in the geology of the country bordering Lake Superior. In the series of papers, of which this is the first, the petrographical characteristics of the various types of these rocks will be described, and the views held by previous workers with respect to their geological relationships will be outlined. Thus, it is hoped, a foundation will be laid for a new and more thorough investigation of the field relations of these rocks than has heretofore been possible. As the case now stands, several of the geologists who have investigated the eruptive rocks of this region have erred in confusing types of entirely different origins, and have thereby introduced into the literature errors of observation that have rendered a clear understanding of the Lake Superior geology almost impossible.

When practicable the laboratory and field study of rocks should proceed together, each aiding the other in solving the knotty problems that so often arise in their progress. The laboraty study of the eruptives in the region under consideration has been almost entirely neglected, and consequently the field problems arising in connection with them have largely remained unsolved. When the peculiarities of these rocks—their composition and structure—become known, much light will be thrown upon their nature, and it will then be time to again review their field relations, when it is believed that many