ONONDAGA-SALT GROUP. 159 



The relations of the masses now under consideration, are well developed on Nine-mile 

 creek, where the following section was obtained : 



Fig. 37. 



1. Green shales. 2. Plaster beds, whose forms are usually oval, and appear like unconformable masses in the shale. 

 3. Porous limestone. 4. Green shales, with their seams of columnar gypsum. 5. Tufa beds, which are con- 

 tinually forming on the brows and at the base of almost every slope where the green shales make the underlying 

 rock of the region. 



Opinions of its origin. From its lava-like structure, some geologists regard this rock as 

 the product of a mud volcano. Perhaps there are no facts which very strongly support 

 this view, or militate against it. The pores, however, are not so much like those of lava, 

 when closely inspected, as they seem to be when viewed only cursorily. They are not 

 such as are usually produced when pent air is expanded in a semi-liquid rock, and forms 

 thereby a small cavity. In these cases there is a smoothness upon the Inner surface, which 

 is due to the pressure of the air upon a soft yielding body. The pores of the limestone 

 are excessively angular or jagged, as if some substance had crystallized in them, and was 

 subsequently dissolved, leaving the open space unoccupied. We may suppose that these 

 porous layers were sediments like the rest of the formation, but contained much crystal- 

 line matter, which has disappeared by solution. Still it is true that mud is ejected from 

 volcanoes, and is spread over wide areas, which, from the laws of soft movable matter, 

 will become, on solidifying, a stratified rock, provided the mass is not fused or exces- 

 sively heated, in which case it would rather become a crystalline unstratified rock similar 

 to granite. 



4. Thin-bedded shaly limestone ; the superior part constituting the manlius 



waterlime group. 



This part of the salt group may be described as consisting of thin-bedded calcareous 

 shales, near the plaster beds, whose colors are usually bluish gray, excepting a few beds 

 which are red or red mottled with green in a manner similar to the red shale at the base 

 of the group. The beds are all thicker above, or superiorly ; but they are always fine- 

 grained deposits, and supposed to be impure carbonates of lime, containing magnesia, iron 

 and alumina. 



This part of the group has been usually described by itself, under the name of Manlius 

 waterlime. As the purposes for which a notice of the New- York rocks is introduced are 



