476 PROF. E. J. GARWOOD ON THE LOWER CARBOmFEROUS [DcC. I9I2, 



to the ty2)ical ' spotted ' limestones found in the lowest beds of 

 this sub-zone. 



There are, however, other ' spotted ' beds showing evidence of 

 the separation of a definite portion of the deposit to form spots, 

 which have a mineral composition different from that of the 

 matrix ; and these beds may throw light on the more obscure 

 cases just described. The best example of this second type occurs 

 in the Melraerby-Scar Limestone of Great Rundal Beck in the 

 Pennine District. In this rock the spots are rather more nodular 

 in character than usual, and the rock has a tendency to weather 

 irregularly, the spots standing out slightly in relief. A fractured 

 surface also shows the same tendency, pointing to the conclusion 

 that the spots are denser and more compact than the matrix. On 

 the other hand, the spots are readily attacked by acid, while the 

 matrix, though easily disintegrated, leaves a large proportion of 

 insoluble residue. In microscopic sections, the mineral composition 

 of the spots and the matrix, unlike the examples previously de- 

 scribed, proves to be essentially different : for, whereas the dark 

 spots are composed, as usual, of fragmentary organic remains, 

 foraminifera, and calcite-crystals, the lighter-coloured matrix 

 is largely composed of subangular quartz-grains embedded in 

 calcite. The matrix also presents somew^hat the appearance of the 

 ' flow-structure ' of a rhy elite (PI. XLYII, fig. 3). In the Melmerby- 

 Scar rock, then, we find definite evidence that segregation of 

 material has taken place, the fragmentary calcareous portions 

 having collected together in patches, while the quartz-grains con- 

 stitute the enveloping matrix ; at the same time, the coloured 

 impurities have also concentrated in the calcareous portions,^ giving 

 rise to the dark spots. 



The study of these different occurrences would thus appear to 

 show that two types of ' spotted ' rocks occur : in the first, which 

 is by far the commoner — where the rock consisted originally of 

 a nearly pure calcareous deposit — the concretionary action was 

 Jiecessarily limited to the concentration of the colouring-matter 

 round centres to form dark spots ; in the second case — where 'the 

 original deposit contained a considerable proportion of sand- 

 grains — a more marked concretionary action took place : the 

 calcareous fragments together with the colouring-matter being 

 concentrated in the spots, leaving the sand-grains to form a pale- 

 grey matrix, in which the darker concretionary spots lie embedded. 

 Though this seems to be essentially what has taken place, the 

 actual process by w^hich the calcareous material in the second 

 case has efiPected a separation from the quartz-grains is not so easy 

 to understand, for the quartz-grains evidently represent original 

 detrital quartz, and nob silica which has been introduced subse- 

 quently in solution. The concentration of one substance in a 

 deposit to form concretions, as in the case of flints in Chalk 

 or the nodules in the Magnesian Limestone of Durham, has 

 generally been regarded as a chemical process affecting material in 

 solution, or at least in a colloid state ; but, in the present case, it 

 would appear to require the actual migration of the foraminifera 



