42 Marker : Petrology in Yorkshire. 



being the hatea or conical pan used by the BraziUan miners. 

 After this, recourse may be had to one of the heavy solutions 

 which have come into use in petrographical laboratories, such 

 as those devised by Klein and Brauns.* These liquids can be 

 prepared with high specific gravity, and diluted down as 

 required. In this way the heavy minerals can be separated, 

 not only from the liglit, but from one another. A further 

 refinement, devised by Professor Sollas, is a> column of heavy 

 liquid with density graduated from top to bottom. In this 

 the grains arrange themselves according to their several 

 specific gravities, and a complete separation is effected in one 

 operation. This method of mechanical analysis may be 

 applied to loose sands or friable sandstones, and equally to 

 those having a calcareous or ferruginous cement, which can 

 be removed by solution in weak acid. By determining the 

 percentages of the several constituents it can be made a quan- 

 titative analysis. It is scarcely necessary to remark that, 

 in work of this kind, isolated observations have but little value : 

 the study should be a comparative one. Thus, by samples from 

 a particular formation taken in different localities, we can 

 trace the change in its constitution as it is followed in a given 

 direction. Such investigations as those of Mr. Thomas on the 

 Trias of the West of England illustrate the results which may 

 be obtained from such a comparative study. It may not 

 unreasonably be expected that conclusions no less interesting 

 would reward a like detailed examination of the Millstone Grit 

 and Coal-Measures of the West Riding, the Trias of the Vale 

 of York, or the Jurassic sandstones of our coast. 



It is not only the accessory constituents of sands that afford 

 a clue to the source of the material. Information may be 

 obtained also from the nature of the minute inclusions 

 in the quartz-grains, and from the shape and surface- 

 characters of the grains themselves. Again, the average size 

 of the grains, the admixture of grains of different sizes, the 

 degree of rounding produced by attrition, these and other 

 characters will often warrant interesting conclusions relative 

 to the conditions of transportation and accumulation of the 

 sediments. Even the humidity or aridity of the climate at 

 past epochs may sometimes be inferred from the state of 

 preservation of the felspar-grains in sandstones. All these 

 are contributions to what is perhaps the most fascinating 

 branch of geology, viz., palaeogeography, the actual restoration 

 of the physical conditions of our area at different remote 

 epochs. 



* Klein's solution is cadmium boro-tungstate, dissolved in water ; 

 Braun's is methyl iodide, diluted as required with benzol. 



Naturalist, 



