436 



E. J. Garwood — Origin of the Concretions 



The great drawback to the evidence is the very variable com- 

 position of those beds in vehich no concretions are developed, and 

 the difficulty of selecting a bed or beds representing the original 

 composition of the deposit, as regards the relative quantities of 

 carbonate of magnesia and carbonate of lime present. In the annexed 

 table are given average analyses of concretionary, stratified, and 

 compact magnesian beds : — 



Column I. gives an average of 12 analyses of the matrix vp^hich. 

 are each tested from 15 samples, the composition shown being thus 

 the average of 180 samples. In the same way Column II. shows 

 the average composition of 180 samples of the concretions. 



The bed selected for examination is that shown in the photograph, 

 (Plate XIII.), which extends for some 12 to 15 feet in a downward 

 direction. It will be seen that whereas the matrix contains 66, the 

 concretions only contain 11 parts of magnesia to 100 parts of lime. 



The overlying bed (No. IV. of the Table) is a well-bedded lime- 

 stone, which forms the surface beds in this district, and if the con- 

 cretions had been formed stalactitically, it is from this bed that the 

 lime must have been derived ; but the bed shows no honeycombed 

 appearance, and is one of the most compact members of the series. 

 This overlying bed, which is 20 to 30 feet thick, is itself thoroughly 

 concretiopary in its structure, although on a minute scale, on the 

 type of the globular concretions which overlie and pass into it, and 

 if the lime forming the concretions is to be regarded as derived 

 from infiltration, the whole of this upper bed must then have been 

 formed in like manner, for we cannot fix an arbitrary size of con- 

 cretion and declare that all those larger than this size were formed 

 by infiltration and all those smaller, although of exactly similar 

 structure, represent the bed as originally deposited. Since then the 

 beds immediately overlying our concretionary bed are also con- 

 cretionary, we cannot consider them as representing (according to 

 the stalactitic theory), the original deposit, and we cannot use them 

 therefore for instituting a chemical comparison with the typical 

 concretionary beds. If, however, we go to higher beds, omitting 

 those which have concretionary limestone interbedded with them, we 

 find at Koker a massive bed nearly 100 feet thick ; its composition, 

 as given by Messrs. Browell and Kirkby/ is shown in Column V. 

 of our Table. 



