232 TRANSITION LIMESTONE. 
fossiliferous groups of De la Beche, and the latter 
might, with as much propriety, be referred to the 
lower secondary formations as placed in the present 
group. 
Transition Limestone, — We have already given a 
short description of this rock (page 82). It is less 
crystalline than the primary, and more so than the 
secondary, being of an intermediate character be- 
tween both. It is generally of a dark gray colour, 
and sometimes black. This colour is owing to car- 
bon and bitumen disseminated through it. It often 
passes into magnesian hmestone, and frequently 
contains alumina. It is not rich in metals, though 
copper is found in it in Virginia. It contains nu- 
merous fossil organic remains, such as of fish, and 
testaceous animals. Of these, the orthocera, the 
encrinite, the productus, the terehratula, and the tri- 
lobite, are the most common. The latter is often 
met with in the transition limestone at Trenton 
Falls in this state. The same formation also con- 
tains numerous coraUiform crystallizations. " The 
transition limestone," says Professor Renwick, " oc- 
cupies a narrow belt of very great length in the 
United States. In North Carolina, Virginia, Mary- 
land, Pennsylvania, and New- Jersey, it lies in a 
valley, often diversified by hills of moderate eleva- 
tion, between the first and second ridges of the Ap- 
palachian group. It enters the State of New- York 
in Orange county, and follows for a time the course 
of the Walkill. Thence it passes towards the 
Hudson River at Newburgh, where it is covered by 
diluvial gravel, but reappears at the surface on the 
eastern bank at Fishkill. From thence to Rhine- 
beck it is wholly confined to the eastern side of the 
Hudson ; here it again crosses the river, and spreads 
to a considerable width in the neighbourhood of 
Kingston and Catskill. At Hudson it occupies both 
banks of the river, and extends eastward for several 
miles, but here finally crosses the river. North of 
