Dr. D. Woolacott — Magnesian Limestone of Durham. 463 



into two types, the spherical and the cellular, which are associated 

 together in the same beds, and similar conditions would appear to 

 have brought them about. The large irregular spherical concretions, 

 the cannon-ball, botryoidal, and stellate types, do not occur on any 

 definite horizons, but are found irregularly through the Concretionary 

 Limestone. It seems probable that the more spherical calcareous 

 growths were produced in parts of the sohition where the colloidal 

 organic matter was more concentrated. While the calcite which 

 forms the concretions is in the main part the interstitial calcite, it is 

 possible that if the rocks were saturated in sulphate solutions part 

 of the dolomite has been decomposed, the magnesium being carried 

 away as a sulphate and the calcite crystallizing along with the 

 interstitial calcite. It is difficult to understand how the large, 

 irregular, spherical (often more than 2 ft. across), non-cellular 

 concretions and the smaller compact cannon-balls can have been 

 formed without such solution having taken place. The matrix and 

 loose dolomitic material surrounding the spherical concretions and 

 found in the cellular concretionary rocks is alwaj-s fine and powdery, 

 and the question arises was the dolomite in these concretionary 

 rocks always powdery and never granular or oolitic. 1 The calcium 

 carbonate in certain of the concretions does not appear to have come 

 together out of the rock, but the change seems to have been brought 

 about by gradual alteration outwards from the centre. Specimens 

 can be obtained varying from soft, yellow, unaltered highly dolomitic 

 beds to rocks in which clearly defined spherules of calcite have been, 

 formed (the dolomitic limestones around these remaining unaffected), 

 such rocks finally by coalescing of the spherules becoming grey, 

 concretionary, calcareous, and only slightly cellular. It is very 

 questionable to me whether this particular change is only due to the 

 crystallization of the interstitial calcite and does not imply solution 

 of the magnesium carbonate. As specimens showing such variations 

 can readily be obtained, observers are inclined to think that changes 

 of this character are taking place at the present time, but this is 

 exceedingly doubtful ; indeed, it is much more probable that these 

 complicated rock-structures are now more or less stable, and that in 

 the main the concretions were produced at some time in the past 

 under the conditions just defined. As Sedgwick pointed out in 1835, 

 the concretions were formed after deposition, and I have shown that 

 they must have been mainly produced before the folding and 

 dislocation of the strata took place. In one case a brecciated 

 mass, which occurs on the coast just north of Byers Quarry, Marsden, 

 has had a crystalline concretionary structure developed in it sub- 

 sequent to brecciation, but this is quite exceptional. Whether any 

 solution of the magnesium content of the dolomite has gone on in 

 these rocks by the action of solutions of calcium sulphate on them 

 or not, it should be noticed that the loose dolomitic powder, which is 

 at present being removed mechanically from them under existing 

 conditions of denudation (and in Durham similar conditions over 

 more extensive areas would hold good before the country was 



1 Trechmann lays emphasis on the very fine state of the powdery material 

 in both the concretionary and segregated rocks (Q.J.G.S., vol.lxx, pp. 252, 256). 



