238 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



also infer that certain forms of Productus which these beds contain 

 were inhabitants of the marginal zone. Though this coal group may 

 not exceed 150 feet in thickness, the fine lines of deposition of the 

 component parts indicate very slow accumulation : the conditions are 

 those of the alternate elevation and depression of a shelving coast- 

 line, which at one time formed a subaerial surface covered with vege- 

 tation, and at another a submerged one, into the waters of which the 

 fallen fern-fronds of some district not far distant were abmidantly 

 conveyed. As the strongest seam at Hardinghen exceeds 3 feet, 

 which would imply an original growth of peat of nearly three times that 

 thickness, the subaerial intervals were probably of very long duration. 

 I am unable at present to assign any definite thickness to the upper- 

 most limestone ; it is greatly reduced by denudation about Leulinghen, 

 and it cannot be readily measured in the Bois des Roches, where it 

 is most complete. The coal-measures are not less than 150 feet 

 thick, but are unequal, and this I have no doubt is due, not to very 

 unequal accumulation, but to the removal of a portion of the series 

 in places before its subsidence into those depths at which the upper 

 hmestone series was accumulated. 



At Haut-banc the limestone is 210 feet thick, but the upper sur- 

 face has been deeply denuded, as may be seen in any of the open 

 workings for iron- ore, in the plateau of Elinghen ; if to this be added 

 the series seen at the black limestone quarry (p. 236), or that near 

 Locquingheu, we may estimate it safely as exceeding 200 feet. 



The course of the streamlet from Beaulieu (which is a line of 

 fault) presents the only clear section of the beds immediately beneath 

 the Haut-banc limestone. 



a. First in descending order are shaly limestones and pure argil- 

 laceous shales, as beneath the limestone of Elinghen. 



b. Next below these is a very considerable mass of stratified dolo- 

 mitic sandy limestone, the calcareous portions at times being small ; 

 the rock is of a dark cindery-brown colour, and harsh to the touch. 

 It contains in parts fragmentary fossil remains, and the disjointed 

 stems and other parts of Crinoidea in abundance. This band seems 

 to belong to the zone of finest drifted sand mixing with calcareous 

 sediment, and it presents external characters which render its recog- 

 nition easy wherever it is exposed. It is seen to the N. of the wind- 

 mill of Le Hure, but is soon concealed by surface detritus ; in a N.W. 

 direction it can be detected at the back of the farm of La Malassise. 

 To the S.E. of Le Hure, it comes out in the Crembreux stream, and 

 crosses the Bois de Fiennes, where it may be seen passing beneath the 

 Haut-banc limestone, as in the road from Fiennes to Locquinghen ; but 

 for this dolomitic band it would be very difficult to fix the age of the 

 limestone at this place, and which passes beneath the coal-basin of 

 Fiennes. 



This dolomitic band is the lowest of the groups of strata which 

 compose the upper division of the Boulonnais series, or that which 

 answers to the "Mountain Limestone" of Belgium and the British 

 Islands. 



