256 ME. C. T. TEECHMANJST 01S" THE LITHOLOGY [June 1914, 



The washing-away of powdery material from the rock entails 

 also the withdrawal of small quantities of calcite, quartz, and other 

 constituents present in it; but the quantity is so small relatively to 

 the dolomite, that the remaining rock is always left in a very much 

 more calcareous condition than before. The somewhat anomalous 

 effect is thus attained, of a partial decalcification resulting in 

 a widespread removal of dolomite and enrichment in calcite 

 of the formation. As already stated, this mechanical removal of 

 powdery dolomite is the only observable process whereby the 

 dedolomitization of the rock on an extensive scale is effected. 

 It is also a very powerful agent in the breaking-down and 

 disintegration of the rock. 



The powdery dolomite is generally so extremely fine, that a very 

 feeble current of water would suffice to carry it through the inter- 

 stices of the rock. In all newly-opened quarries or sections in 

 segregated or cellular beds, the powder can be seen in process of 

 removal. A shower of rain washes' it in quantities out of the rock,, 

 after which it hangs on the rock-face in large dried masses. The 

 small earth-tremors often felt at Sunderland and elsewhere on such 

 beds are attributed to settling-down consequent upon this removal. 



V. The Origin of the Celltjlae Stbuctubes. 



The true cellular structures are most conspicuously developed, as 

 already stated, in the cliffs at Trow Rocks near South Shields. 

 They also occur on a much smaller scale at several localities on the 

 eastern slope of the Shell-Limestones (Ford Quarry, Easington 

 Colliery, Down Hill, etc.), where the reef merges into the bedded 

 equivalents on its flank. Every gradation can be traced from the 

 Bryozoan Reef -dolomite, through the cellular rock, to the calcareous 

 segregated fragments which have suffered collapse and brecciation, 

 consequent on more or less complete removal of powdery dolomite. 



Before the origin of these structures is discussed, a few of the 

 outstanding features observable on a critical examination of them 

 may be noticed : — 



(1) The cementing material is alwaj-s more calcareous than the enclosed 



fragments, and is very inconstant in composition. 



(2) The fragments are angular, and quite irregular in shape and size. 



(3) Bedding or lamination is often apparent in the angular fragments, or 



is seen on the sides of the cavities whence the powdery material has 

 been removed. 



(4) The interior walls of the cells, when emptied, are generally seen to 



have a rough and irregular surface, as though a later deposit of 

 calcite-grains derived from the enclosed fragment had taken place 

 upon them. 



(5) The powdery material differs in no way from that associated with 



other forms of calcareous segregations, except that, being enclosed 

 in a cell, it has no ready means of escape. 



(6) I have never seen what has been termed a ' negative breccia.' On 



exposed surfaces and near cracks and fissures, the powdery dolomite 

 is generally removed completely from the cells ; but, on freshly- 

 broken surfaces, the cavities are always either loosely or compactly 

 filled with powder. 



