246 AMADEUS W. GRABAU 
to 700 feet at the Delaware water gap, to 400 feet at the Lehigh, and to 
less southward. The thinning is by failure of the lower beds, showing 
this to be a true non-marine overlap, and therefore stamping the 
series as of river origin. The Eurypterid layers in the upper beds 
are probably contemporaneous with the basal Eurypterid beds of 
the Salina of New York. The succeeding series of Longwood 
shales resembles the Juniata-Queenston, and, like it, has all the ear- 
marks of a continental series formed under semiarid conditions. 
They thin from 2,385 feet in New Jersey to 120 feet at Cornwall, 
75 feet at High Falls (High Falls shale); and 25 feet at Rosendale, 
and disappear farther north. Southward they thin likewise, while 
westward only the upper 400 feet of the series is shown in the red 
Lower Salina shales of Ithaca and Syracuse, New York, where they 
are succeeded by salt deposition and less than 200 feet at Buffalo. 
Like the conglomerate, the shales thin by failure of the lower beds, 
i. e., by non-marine overlap away from the source of supply. 
F. THE UPPER SILURIC OR MONROAN 
This is typically developed in southern Michigan, Ohio, and west- 
ern Ontario, where it is divisible into Lower Monroe or Bass Islands 
series, 500 feet thick, or more; middle Monroan or Sylvania sandstone 
30 to 150 feet thick; and Upper Monroan or Detroit River series, 
300 to 400 feet thick." The entire series is involved in gentle folding 
of early Devonic age, the Dundee resting upon the eroded surfaces 
of various members of the series. : 
The Lower Monroan represents an invasion from the Atlantic 
across Maryland, Pennsylvania, and southern Ohio, to Michigan and 
probably Wisconsin. Western Ontario was involved, but apparently 
not western New York. ‘The fauna is Upper Siluric, genetically 
related to the Manlius limestone fauna, and, like it, representing an 
Atlantic type. The Upper Monroan fauna, on the other hand, is of 
a distinct type, especially in the lower members (Flat Rock, Amherst- 
burg, and Anderdon beds). Besides being related to the later Nia- 
garan fauna, it has a new coral and brachiopod element suggestive 
t See Sherzer and Grabau, Bull. Geol. Soc. Amer., Vol. XIX. The full dis- 
cussion of these formations and their fauna will appear in the report of the Michigan 
Survey. 
