36 The American Geologist. January, 1895 
It may be assumed therefore that of the 100 species known in 
the Utiea 25 originated in the Trenton, and of the 78 species 
known in the Galena 56 originated in the Trenton or in a 
lower horizon. According to that the ratios of alliance may 
be expressed thus: 
Eatio of Trenton species found in the Utica, in all parts of 
the country, 25 per cent. 
Ratio of Trenton species found in the Galena, in the upper 
Mississippi valley alone, 70+ per cent. 
The table therefore shows a closer alliance of the Galena 
with the Trenton than of the Utica with the Trenton. Mr. 
Walcott remarks : 
The table shows a greater change took place in the fauna of the Utica 
slate than in that of the Galena limestone, the former having fifty-four 
species limited to its boundaries, and thirtj'-six derived from the Tren- 
ton; while the latter has nineteen species peculiar to it, and fifty-six 
passing up from the Trenton formation beneath. 
Mr. Walcott's data therefore, in this respect, do not strongly 
support his own conclusion as to the occurrence of a break at 
the top of the Trenton, separating it from the Galena. On the 
contrary his data would seem to indicate a strong connection 
between the Trenton and Galena. When it be considered fur- 
ther that he adduces no evidence whatever of a lithological 
change at that horizon, in the region of the Upper Mississippi, 
but that all his quotations bearing on the top of the Trenton 
are descriptive of the passage to the Utica slate in other por- 
tions of the country, or, when they apply to the Trenton of 
the Upper Mississippi, they testify to the slowness of the 
change from the Trenton to the Galena, it appears that the 
conclusion announced is hardly supported by the evidence 
adduced. 
Recent work on the rocks of the Lower Silurian by the 
Minnesota Geological Survey has brought to light numerous 
adverse facts bearing on this question, which, put against the 
paucity of affirmative evidence adduced by Mr. Walcott, lay 
at rest forever all uncertainty of the age of the Galena lime- 
stone. Messrs. E. O. Ulrich and Charles Schuchert have co- 
operated with the writer in studying the paleontology of the 
Lower Silurian in the Upper Mississippi valley. The infor- 
mation in detail will appear in vol. iii of the final report of 
the survey, now in press. It is sufficient here to give a part 
