214 Dr. Gilby on the Magncsian Limestone , Esfc. 



tion in structure in an individual formation, has not so much been 

 derived from the addition or substraction of certain chemical in- 

 gredients as from the proportion in which these ingredients have 

 crystallized. According to the latter view, during the consolidation 

 of any particular formation, the constituent particles, although few 

 in number, may in different parts of the crystallizing mass have been 

 attracted together in new proportions, so as to give rise to those 

 variations in colour and structure which we so frequently witness. 

 The originality of this theory of crystallization belongs to Professor 

 Jameson, and it seems to me very happily to explain many anoma- 

 lous appearances of disorder and brecciated structure, which have 

 caused great embarrassment to geologists. In some cases however 

 this theory cannot be applied with any degree of probability. Where 

 we see a particular assemblage of strata, as the limestone conglome- 

 rate, manifestly of the same formation, exhibiting in several parts of 

 its extent changes of composition altogether depending upon a dif- 

 ference in its chemical constitution, it is impossible to explain such 

 an occurrence but by supposing that the fluid menstruum must have 

 contained in different places different chemical ingredients. Every 

 geologist will figure to himself illustrations of the want of uniformity 

 in the same rock formation. I may mention however two other 

 striking facts of this nature. The red clay of the red ground is met 

 with in almost every part of England, and almost every where does 

 it contain or is connected with gypsum ; but besides gypsum, in this 

 neighbourhood only, it abounds with sulphate of strontian in the 

 form of veins and even large beds. From Mr. Webster's account 

 of the strata above the chalk in the Isle of Wight, it seems quite 

 manifest that what he calls the first fresh water formation was 

 formed at the same period with the marl and gypsum of the Paris 



