THE SOUTH-EASTERN COALFIELD 401 



pairs of strong lines of nodular flints. This marl band is the 

 base of the Upper and summit of the Middle Chalk. 



Fossils characteristic of the whole zone are a special form 

 of Micraster cor-tcstudinarium and Micraster precursor, Holaster 

 planus, the gibbous variety of Echinocorys scutatus, Micraster 

 leskei, and M. cor-bovis. 



Zone of Terebratulina gracilis. 



The chalk of this zone is massive and firm, with few open 

 joints, and grey with admixture of marl. It is consequently 

 relatively impermeable to the percolation of water, which finds 

 a line of least resistance in the marl bands which occur at 

 regular intervals : these marl bands form false spring planes, 

 and in the sinkings water is not met with in the chalk itself 

 in this zone, but small feeders are generally thrown out by 

 the marl bands, which occur regularly at known intervals. 

 Flints have become rare. 



This zone is 161 ft. thick. Fossils are not numerous, but 

 Terebratulina gracilis itself ', like a pretty little scallop, is common, 

 especially in the marl seams. The primitive Micrasters, Micraster 

 leskei and Micraster cor-bovis, occur near the top of the zone, 

 but are rare, and fragments of a big coarse-ribbed, convex 

 bivalve called Inoceramus lamarcki Parkinson (= brogniarti 

 Mant.) are common. 



Zone of Rhynchonella cuvieri. 



The passage between these two zones is taken on the coast 

 at the lowest line of flints, but this does not hold good inland. 

 The real passage is a zoological one, and is marked by the 

 replacement of Inoceramus lamarcki by the elongate, depressed, 

 finely striated, mussel-like Inoceramus mytiloidcs (= labiatus 

 Schl.) ; Rhynchonella cuvieri, a little convex ribbed cockle, which 

 first appears at the base of the Upper Chalk, now becomes really 

 common, but other fossils are not numerous. 



The zone is 70 ft. thick : the upper half is scarcely distin- 

 guishable from the zone above in physical characters. The 

 marl is present not so much in clear, well-marked bands as in 

 ill-defined and very numerous streaks and veins. But about 

 the middle, bands of hard nodules appear, and the bottom 

 32 ft. of the zone consists of an indurated rough nodular chalk 

 known as the Grit Bed, which corresponds to the Melbourne 



