LOWER SILURIC SHALES OF THE MOHAWK VALLEY 7 



HISTORICAL SKETCH 



A perusal of the literature shows that the Utica and Frankfort 

 shales have received scanty attention from geologists, quite obvi- 

 ously because they promise little, if any, results, the Utica shale 

 fauna consisting largely of small species difficult of study. The 

 Frankfort shale was declared by Vanuxem to be remarkably deficient 

 in all organic remains excepting graptolites. A fairly full account 

 of the two was given by Vanuxem in the Geology of the Third 

 District. This astute observer distinguished the Utica slate and the 

 Frankfort slate; the latter as a subdivision of the Hudson River 

 group or Lorraine (Emmons), the other member being the Pulaski 

 shale. He states that in his district (which included the Mohawk 

 valley) the Utica slate is everywhere underlain by the Trenton 

 limestone and overlain by the Frankfort shale and sandstone; that 

 both the Utica and Frankfort beds enter his district from the first 

 district on the lower Mohawk and are coextensive there, while 

 the Pulaski shale first appears much farther west in Oneida county. 

 The Utica slate is characterized as deep bluish black rock, generally 

 fissile, exhibiting a brownish or dark chocolate color by weathering 

 and associated with thin layers or flags of dark colored impure 

 limestone, especially in the lower part of the mass. As character- 

 istic fossils, that do not come up from the Trenton, are cited : 



Triarthrus becki, Graptolites den tat us 

 (either Dipl. foliaceus or Glossogr. quadri- 

 mucronatus), a mutation of Trocholites ammo- 

 nius and Graptolites scalaris (Diplograptus and 

 Climacograptus viewed from in front). 



The Frankfort slate is stated by Vanuxem to change by imper- 

 ceptible gradation from the Utica slate, " yet a separation would 

 seem obvious from the fact that the dark blue or black color of the 

 latter disappears in the usually light color of the former. While 

 the Utica slate alternates at its lower part with thin beds of dark 

 colored impure limestone, the Frankfort slate, on the contrary, 

 alternates with a peculiar sandstone to which Professor Eaton gave 

 the name of rubblestone ; and while the Utica slate is calcareous, 

 the Frankfort slate is wholly destitute of calcareous particles," He 

 continues : '' Both slate and rubblestone " (greywacke of Eaton, an 

 impure argillaceous sandstone), "lose their bluish color when long 

 exposed, and assume a dull dark gray, green or olive color, which 

 is very characteristic of this rock, and by which it is readily 



