302 ME. A. STEAHAN ON THE PASSAGE OF A [Allg. I9OT, 



motionless water. Clastic material is extremely scarce, and when 

 present (E 3280 : PI. XII, fig. 2) is so fine as to be recognizable 

 with difficulty. In many of the specimens there is practically no 

 mud. The vegetable debris are finely divided, and even after 

 mineralization take long to settle ; before mineralization they must 

 have been in a condition to travel with the smallest movement in 

 the water, Por the dolomite no more current would be required 

 than would suffice to bring to the spot fresh supplies of the car- 

 bonates in solution. The irregularity of the structures and the 

 absence of lamination point also to tranquillity ; the irregularities 

 of the surface were accentuated rather than smoothed over by the 

 successive layers of crystalline and coaly matter. 



The conditions, therefore, were those under which a tufa would 

 be formed — a mode of origin to which the structure and character 

 of the rock also point. The formation of tufa is generally confined 

 to shallow water, and is attributed theoretically to chemical action. 

 More frequently it is due probably to the absorption of carbon- 

 dioxide by alg£e, and to the reduction of the bicarbonates to the less 

 soluble carbonates. In the case under consideration the presence 

 of vegetable matter would tend to prevent the precipitation, if 

 carbonic acid were evolved ; but from a general consideration of the 

 character of coal-seams there is reason to think that the vegetable 

 matter of which they consist was preserved from decomposition by 

 being wholly submerged. There is no reason to doubt that the 

 shreds of vegetation which reached the dolomitic region were fully 

 submerged. 



The presence of magnesium-carbonate is somewhat unusual in 

 tufas. The fact that the measures are, or have been, overlain by the 

 ISTew Red Sandstone suggests the possibility of the tufa having been 

 originally calcareous and subsequently dolomitized, but limestones 

 lying immediately below Triassic beds often show no dolomitization. 

 On the other hand, Coal-Measure waters themsehes are strongly 

 mineralized with salts of lime, magnesia, iron, and less commonly 

 of baryta and soda. It seems, therefore, more probable that the 

 composition of the tufa was due to the character of the water in 

 which it was formed, than to the subsequent introduction of carbonate 

 of magnesia. 



The supposition that the deposit marks the site of a spring of Coal- 

 Measure age is negatived by a consideration of the physical conditions 

 of the period. There is evidence that the coals were formed at or 

 below sea-level, and that there was -certainly no high ground, and 

 probably none above sea-level, within some miles of the spot. 

 Under the circumstances th^re would be nothing to force the water 

 up from the strata underlying the seam. 



A coal-seam may be regarded as the last phase of an episode of 

 sedimentary activity. The Coal-Measures, though on the whole a 

 most irregular deposit, are built up of repetitions of a certain definite 

 sequence of deposits, sandstone or conglomerate being succeeded by 

 shale, shale by coal. By a repetition of this sequence each coal 



