SPEXCER : THE HALIFAX COAL STRATA. 
305 
less in most of the shale beds from the top to the bottom of the 
Coal-Measures. Sometimes the small Spirorbis flourished in such 
incalculable numbers as to form beds of from 1 yd. to 6 yds. in 
thickness of impure limestone. Anthracosia and Spirorbis are char- 
acteristic Coal-Measure fossils, and the latter has been frequently 
found attached to the stems of fossil plants and the fronds of ferns. 
The Anthracosia were at first called Unios from their 
resemblance to those of the recent genus Unio. But afterwards, on 
account of their being often found in the debrU on our coal-pit 
banks, along with Goniatites and other marine shells, it became 
doubtful as so whether they were of freshwater or of marine origin, 
and so they were given the name of Anthracosia, which simply 
means " Coal- Measure Shells." 
In Yorkshire we have never had any doubt upon the point, 
having always regarded them as being of freshwater or of estuarine 
origin, and not long ago, my old friend, Mr. George Wild, one of the 
best practical geologists in Lancashire, informed me that the point 
had been clearly settled for that county, viz., that Anthracosia do not 
occur in situ in the same beds as the marine shells, and that there 
is now no reason for doubting their being of freshwater origin. 
S2nrorbis carhonarius appears also to have been a freshwater species 
belonging to the Annelid family which, at the present day, is well 
represented by marine species infesting the sea-weed on our coasts. 
After about 30 ft. to 40 ft. of shale had been deposited, a bed 
of fine gritty sand was formed (Middle Band Stone) of from 3 ft. to 
5 ft. in thickness, which is now one of the hardest rocks in the 
whole group. This was covered by a bed of seat-earth, then the 
area became a land surface, and the vegetation which formed the 
Middle Band Coal was produced. Afterw^ards the land once more 
subsided beneath the waters and about 30 ft. of shales Avere 
deposited upon the coal, followed by 4 ft. to 6 ft. of seat-earth and 
Gannister Rock. This gannister is a hard, fine-grained silicious 
rock, full of Stigmarian rootlets, and occurs in the midst of the 
seat-earth. It is one of the most characteristic rocks in the 
Carboniferous formation, and is closely allied to the Galliard Rock 
of the Millstone Grit series. 
