in structure in an individual formation, has not so much been derived from the addition or substraction of certain chemical ingredients 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 anomalous 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 conglomerate, manifestly of the same formation, exhibiting in several parts of its extent changes of composition altogether depending upon a difference 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
