464 Notices of Memoirs — British Association — 



abundant in this area. Though the dominant colour is grey, the 

 rock is often buff and occasionally almost white. 



The purest form of Spirorbis limestone occurs in masses of very 

 variable size. The largest and most persistent bed is the Index 

 limestone, which occurs roughly about 100 feet down in the 

 Halesowen or Newcastle Group. This has often been confused with 

 another and less persistent bed, lying about 100 feet further up and 

 close to the base of the Keele Group. Other and less persistent bands 

 have been met with in the Keele Group, notably by Mr. Cantrill. In 

 addition to these distinct beds, which can often be traced for some 

 distance at the outcrop, if the grgund be free of drift, there are 

 lenticles varying in length from a few yards to a few inches, and at 

 times only scattered nodules. These smaller patches were found 

 during the great drought, when the old marl pits in the Halesowen 

 Group were completely dried up. Advantage was taken of this to 

 clean the pits out, laying the rock sides bare, when these minor 

 occurrences of the limestones were exposed. 



The limestones seem to have been built up of a series of films or 

 layers, resulting from the evaporation of shallow sheets of lime- 

 bearing water. When dried the film appears to have been cracked 

 and more or less broken up, but re-cemented by later deposits of 

 identical material ; this in turn became broken up and re-cemented. 

 The process was repeated till a bed several feet thick was at last 

 accumulated. The whole rock thus comes to have a clean sharp 

 fracture, though its fragmental character is easily seen on a freshly 

 fractured face. In this form, best shown by the Index limestone, 

 there is a minimum of material other than lime brought into the 

 deposit. A rough test of the brecciated original fragments shows 

 the limestone to be nearly pure and containing about 95 per cent of 

 carbonate of lime. 



From this we pass to the type containing small fragments of other 

 material, such as marl, and the cementing matrix is not merely 

 calcite, a considerable proportion of mud and sand being present. In 

 this the limestone fragments are somewhat rounded, having been 

 transported for short distances. At times the fragments are locally 

 heaped up and the bed attains a quite abnormal thickness. The band 

 at or near the base of the Keele Group shows this character in the 

 cutting of the mineral railway above Kingsbury ; the fragments have 

 been heaped up till it has locally attained a thickness of 10 feet. 



The extreme type is really a cornstone, or a sandstone more or less 

 crowded with rolled fragments of Spirorbis limestone. It is doubtful 

 in this case if any of the rounded fragments are formed in situ ; the 

 whole rock seems to have been the result of flood action tearing up 

 a deposit cracked by drying and transporting the fragments for 

 some distance. 



There is strong evidence to support the view that two at least of 

 these limestones were formed over a large area ; the Index has rarely 

 been removed completely by this process ; the one next above often 

 has. How far the less persistent beds have been locally removed by 

 subsequent erosion is at present an open question. 



This mode of origin of the more impure, possibly of all the 



