Page:The Journal of geology (IA journalofgeology11893univers).pdf/104

to attempt to summarize this discussion, since it is as brief as is consistent with adequacy, in its original form. Suffice it to say that while, as applied to Paleozoic rocks, the value of lithological characters and structural relations are well understood, they have a somewhat different meaning and a greater relative value when applied to the pre-Cambrian formations. At the same time this application is more difficult.

One of the most valuable parts of the volume consists of the twelve maps, coverning most of the areas where pre-Cambrian rocks are known or suspected. Nowhere else does Professor Van Hise succeed better in making the indefiniteness of our knowledge definite, than on the maps. On but two of the twelve maps does he represent Archean rocks, viz., on the maps covering the original Huronian area and its surroundings, and on the map of the Lake Superior region. Within the United States, Archean rocks are mapped in but three states—Minnesota, Wisconsin and Michigan. This does not mean that Archean rocks do not exist elsewhere, or that they are not known elsewhere, but that their areas elsewhere, so far as covered by the maps, have not been defined. Some of the areas which we have been accustomed to see represented as Archean on maps made before the Algonkian was differentiated, are now represented as "unclassified pre-Cambrian." Of this the Adirondack region may serve as an example. The maps tell us only that the rocks of this region may be Algonkian, or Archean, or both. In the text Professor Van Hise's opinion concerning the area may be found. This is to the effect that the Algonkian is certainly represented in the region, and Archean possibly, but that existing knowledge on the point is not sufficiently definite for cartographic representation. Other areas which have been mapped as Archean are represented simply as "unclassified partly or wholly crystalline rocks." Of the areas thus represented, the whole of the crystalline schist belt of the Appalachian region may serve as an example. The author's map does not even assert that these rocks, or any part of them, are pre-Cambrian. Here again we find the author's opinion in the text, where it is indicated that parts of this area are pre-Cambrian, while other extensive portions may, or may not be. Such pre-Cambrian areas as are known are not defined, and therefore cannot be represented on the maps.

Algonkian rocks find definite representation in more regions than the Archean. They appear upon the maps in Arizona, New Mexico,