Page:The Journal of geology (IA journalofgeology11893univers).pdf/112

interlaminations of limestone with shale or sandstone at the transition horizon which frequently occurs when a limestone formation rests upon a sandstone formation.

As to the age of the Keweenawan, this series is placed by Dr. Wadsworth as a lower part of the Potsdam, but is regarded by the reviewer as resting unconformably below the Potsdam, and as belonging to a different geological period. This question is one of great complexity, which can not here be discussed in detail. However, Dr. Wadsworth refers the Keweenawan so doubtfully to the Potsdam that the difference can hardly be said to be a serious one. The statement that the most probable explanation of all the phenomena at Keweenaw Point is that the Eastern sandstone is of different ages can have but one meaning—that a part of this so-called Eastern sandstone belongs to the Potsdam, and this Potsdam is later than, and unconformably upon, the Keweenaw series, which latter includes another part of the Eastern sandstone. Put in another way, Dr. Wadsworth extends the term Eastern sandstone to cover all of the sandstone exposed until the Traps are reached. That is, the break between the Potsdam and Keweenawan is in places a short distance away from the Traps. This admits the difference in geological age between the main area of Potsdam sandstone and the Keweenawan, and merely shifts the boundary line between the two a short distance. It is notable that the most important new evidence presented upon the question is that obtained by Mr. Seaman, Dr. Wadsworth's assistant. Near the South Range he finds outcrops which he regards as Eastern sandstone, holding indurated fragments derived from adjacent ledges of upper Keweenawan sandstones, and hence believes the Eastern sandstone to represent a later geological age.

It appears to the writer very doubtful whether the large number of members given for the Republic and Holyoke series will be found to be general for the Lower Huronian and Upper Huronian on the south shore of Lake Sueprior, although each may be found at some locality.

Wadsworth^[1] states that recent work renders it probable that the Azoic or Archean of Northern Michigan is divisible into five unconformable formations. The tentative arrangement, commencing with the oldest, with the parallel formations, as determined by the United States Geological Survey, is as follows: