J. W. Jackson — Foraminiferal Limestone. 267 



The matrix is light-coloured and fine-grained, with irregularly shaped 

 patches (? nodules) of a fine-grained dark-coloured limestone. The 

 patches have no definite shape, heing mostly very uneven in size, 

 some of them being long cylindrical bodies with irregular contours. 

 The patches are in no case confluent, and, like the Derbyshire examples, 

 there is a tendency for them to break along the line of contact with 

 the lighter portion. When exposed to the weather for any length of 

 time, and along the numerous cracks and fissures in the quarry, the 

 rock weathers in the same peculiar manner as the examples above 

 mentioned, that is, the dark patches resist the weathering to a greater 

 degree than the lighter portion of the rock, and the patches stand 

 out in relief similar to the encrinital limestone of some parts of 

 Derbyshire. 



In the Silverdale specimens, so far as I have made out, foraminifera 

 are almost as numerous in the light portion of the rock as in the dark, 

 which contrasts strongly with the specimens from ])erbyshire, where 

 the light part is said to be almost void of these remains. The same 

 state of things exists in the Boulder-clay specimens. Other organic 

 remains such as mollusca and corals are rare in the rock. 



!Now as to the physical conditions which led to the formation of 

 this mottled limestone. Messrs. Barnes and Holroyd in the above 

 cited paper give as their opinion that the dark patches have been 

 removed from the original place of deposition, and intermingled and 

 cemented into the lighter matrix. They further state that — "In 

 the discussion upon a former paper read by us at the March meeting 

 upon the pebbles found in the Windy Knoll conglomerate. Professor 

 Boyd Dawkins threw out the suggestion that the pebbles might 

 possibly be patches of a partially hardened calcareous mud from the 

 sea-floor, and this idea would seem to ofi'er an explanation of the 

 presence of the dark patches in the light matrix, and it is not difficult 

 to believe that their origin is due to one of the volcanic outbursts 

 that are found represented iu the limestone, for upon the same or 

 a verjr near horizon, as far as we are able to make out, there exists 

 a bed of volcanic rock. Given that these outbursts of volcanic activity 

 were preceded by the usual earth shocks and sea disturbances, and 

 the neighbouring presence of calcareous beds of dark foraminiferal 

 mud, we have all the conditions necessary for the turning up from 

 the sea bottom, and consequent removal to another area of the dark 

 parts, together with the redeposition and consolidation, to produce 

 the peculiar mottlings." 



This highly interesting explanation of the mottling of the limestone 

 appears to hold good with regard to the Derbyshire beds where there 

 have been contemporaneous lava-flows (e.g. Toadstone) along with 

 the dejDOsitioa of the limestone ; but how are we to account for the 

 same phenomenon of mottled limestone so far away as Grange and 

 Silverdale, where no contemporaneous igneous rocks are known to 

 exist, unless it can be put down to the action of strong currents 

 sufficient to remove the deposit from one part of the sea-floor to 

 another, set up by the volcanic disturbances in the Derbyshire area, 

 or can the area of these disturbances have been more widespread than 

 is at present known ? This is certainly a most interesting problem. 



