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result in extending the space taken by the comments beyond that required for the summaries. However, when the points at issue are of general interest, or fundamental importance, it is advisable to make comments and enter into discussions, even if the space taken by such comments be greater than that given to the summary of the original articles. In such comments neither commendation nor censure will be made, but the aim will be to point out the conclusions announced which fail of complete establishment, and the generalizations which appear to go beyond what is warranted by the facts published. The purpose of indicating what appears to the editor as deficiencies of these kinds is neither to put himself dogmatically in opposition to the statements of the author reviewed, nor with the belief that his opinion has more weight, but to direct attention to the questions involved, and in cases of doubt to keep them open for farther study in the field and laboratory.

Mills^[1] finds in the Sierra Nevada, unconformably below the Mesozoic, eruptive granites and sedimentary slates and quartzites. The latter in places rest and were probably deposited upon the granite, while in other places they are contemporaneous and imbedded within it. The quartzites are held to be silicified phases of the slates. These rocks in age may run from Archæan to the Paleozoic and some of them may be early Mesozoic.

Darton^[2] finds Ordovician fossils in the crystalline slates and schists of the Piedmont plain of Virginia, these rocks having been previously regarded as Huronian.

Lesley^[3] gives a summary sketch of the pre-Cambrian rocks of Pennsylvania, the facts being taken from the detailed state reports. The Highland Belt of New Jersey and Pennsylvania; the Reading and Durham Hills; areas in Chester, Bucks, Montgomery and Delaware counties; and an area on the Schuylkill river are placed in the Archean. All are regarded as sedimentary in origin, because of the presence of marble, apatite and iron ore. The newer gneiss of the Philadelphia belt, the Azoic formations of York, Chester and Lancaster counties, and the South Mountain rocks are not definitely referred to any system. The term Huronian must be used simply as a proper and private name for a series of rocks exposed along a part of the northern boundary of the United States. Should a similar series appear in some other region and be called Huronian on account of the resemblance, the name would have no value whatever; unless we should imagine that in a so-called Huronian age the whole surface of the planet was stuccoed with a certain formation; and