Mr D. A. Wells on the Origin of Stratification. 291 



indigo in the sulphuric acid, not only is a part of it liable to 

 escape solution, and proper estimation, but, in the case of 

 inferior indigo, sulphurous acid may be produced, which would 

 of course involve a larger consumption of the bichromate of 

 potash than the indigo-blue itself would require. It may also 

 be objected that bichromate of potash, in the presence of hy- 

 drochloric acid, will act upon the other constituents of ordi- 

 nary indigo ; but, so far as I have been able to judge from a 

 very extensive course of experiments upon a great variety of 

 specimens, the amount of these influences is extremely slight, 

 and altogether inappreciable when the process is executed 

 with proper care. The same opinion has been expressed by 

 Eerzelius and Schlumberger regarding the chlorine process ; 

 and it is further supported by the fact, that indigo contain- 

 ing a large proportion of brown and other colouring matters, 

 consumes a very small quantity only of the bichromate. While, 

 therefore, this process has no pretensions to supply scientific 

 men with the means of determining the actual amount of pure 

 indigo-blue in samples of commercial indigo, it is, in my opi- 

 nion, admirably adapted for ascertaining their relative values, 

 being in many respects superior to those which have hitherto 

 been proposed. 



The bichromate of potash possesses, in an eminent degree, 

 all the qualities requisite for a trustworthy agent of valua- 

 tion, being easy of purification, unchangeable by keeping, and 

 of uniform composition. 



On the Origin of Stratification. By D. A. Wells, Esq., of 

 Cambridge, United States, North America. 



The general idea respecting the origin or cause of stratifi- 

 cation as expressed in geological text-books, or as inferred 

 from the writings of geologists, seems to be this : that strata, 

 or the so-called divisions of sedimentary matter, have been 

 produced either by an interruption of deposition, or a change 

 in the quality of the material deposited. This idea is well illus- 

 trated by the deposition of matter by tides or inundations, its 

 subsequent consolidation, and a renewed deposition on the 



