150 Geology of Upper Illinois. * 
alternations of black bituminous shale, with a dark gray, friable, 
slaty marl, the series being surmounted by a heavy bed of eneri- 
nal limestone. The shale is in beds of between three and four 
feet in thickness, while the clayey mari-strata are considerably 
thicker. The shale-stratum next the coal, embraces a layer of 
limestone about ten inches thick. Large balls of limestone also, 
of a very peculiar appearance, are common throughout the shale. 
They may be described as flattened spheroids, extremely reg- 
ular in shape, smooth, and of a black color. They are arranged 
between the layers of the slate, with their flat surfaces coinciding 
with the stratification. Veins of calcareous spar, tinged brown 
by petroleum, divide their surfaces off into quadrangular and pen- 
tagonal shapes, thereby imparting to the balls a tolerable resem- 
blance to certain tortoises, petrifactions of which animals they are 
considered to be, by many people of the neighborhood. In some 
instances, these balls, which are in reality a species of septaria, 
have a diameter of between two and three feet. The dip of the 
bed at this place, is about 10° or 12° to the W. 
The rock on which the coal rests, as may be seen a little higher 
up the river, is a light gray, highly crystalline limestone. It oc- 
casionally embraces small seams and irregular shaped masses of 
calcareous spar, and is generally so rich in bituminous matter, as 
to afford the odor of this substance on friction. But three fossils 
attracted my notice in it : these were a trilobite, (a species of Caly- 
mene,) a F'lustra, and a Producta, which Fig. 4. 
so closely resembles a pecten in general fig- gaaeee 
ure, as well as in the delicacy and distinct- 
ness of its ribs, (56 to 60 in number,) that, 
believing it to be new, I shall call it the P. 
pectenoidea. (Fig. 4.) The two latter fos- 
sils are very common. Of the tribolite I 
saw but a single sample, and that was presented me by Rev. Mr. 
Euutor, of Vermilionville. The bluffs on the east side of the 
river, in the vicinity of Elliot’s dam, abound in the relics of the 
spontaneous combustion of coal, such as hardened slate and de- 
tached grains and crystals of gypsum, mingled with clay and 
marl. It is in the bed of the river near this place, also, that sev- 
eral mineral springs occur, a more particular notice of which will 
hereafter be given. 
