ANALYSES OF SOME CAMBRIAN ROCKS. 383 



Bischof is in favour of removal by solution in the wet way, and 

 instances the very large quantities of phosphates of lime and of 

 alumina in the Carlsbad waters. Still he admits that phosphoric 

 acid is more plentiful in recent than in old lavas, though the latter 

 more frequently contain apatite. This looks very much as if the 

 wet way tended in some cases to deprive igneous rocks of their 

 phosphates. Thus it may be that this trap at one time contained 

 more phosphoric acid than it now does; for the whole rock is 

 evidently in a very altered condition. The outside is largely mixed 

 with calcite, whilst the interior (see Analysis No IV.) is half made 

 of a hydrated silicate, soluble in acid, of which the principal bases 

 are ferrous oxide and magnesia, pointing in the direction of ser- 

 pentine. 



In this case it must be admitted that there are not sufficient data 

 whereon to base correct conclusions ; but a more detailed examination 

 of the various points of contact between igneous and sedimentary 

 beds, together with careful analyses of each, both close to and at a 

 distance from the point of contact, might help towards the formation 

 of a reasonable hypothesis which should explain some of the pheno- 

 mena at present so puzzling. 



Discussion. 



Prof. Maskelyne said that the solution of the question of the 

 diminution of the phosphate of lime near intrusive rock was easy. 

 The intrusive rock in cooling would contract, and thus facilitate the 

 percolation from above or the forcing up from below of water, which, 

 flowing through the adjacent rocks, would dissolve out the phosphate 

 of lime. He remarked that north of Cardigan Bay pisolitic iron-ore 

 was found charged with phosphate of lime. 



Mr. D. Forbes stated that when, many years ago, Dr. Daubeny 

 maintained that the Cambrian rocks only contained mere traces of 

 phosphoric acid, he had published (in 1857) chemical analyses of 

 some of the oldest limestones then known, showing that these con- 

 tained quite as much phosphoric acid as recent ones. He considered 

 that the larger proportion of phosphate of lime found in the fossil 

 Crustacea, as compared with the recent, was mainly due to the fact 

 that, besides the organic matter, carbonate of lime had also been 

 removed from them, and that the reason of the casts of Paradoxides 

 near dykes of igneous rock containing only traces of phosphoric acid 

 and lime was rather the removal of these substances by water than 

 the igneous action of the dyke. He could not agree in believing 

 that the phosphoric acid found in eruptive rocks was derived from 

 the sedimentary fossiliferous beds through which they passed, but 

 regarded it as an inherent constituent of the eruptive rock itself. 

 Even the Menevian and Laurentian rocks were, after all, only made 

 up of the debris of previous eruptive rocks ; and he looked upon the 

 eruptive rocks as the original source of all the phosphoric acid 

 assimilated by animal and vegetable life. He regarded this paper 

 as a most valuable contribution to geological science. 



Q, J. G. S. No. 123. 2 d 



