Abstract
"Of what materials is the Earth composed, and how are those materials arranged?" Such is the brief but most pithily worded proposition with which the late Sir Charles Lyell commences the first edition of the small duodesimo bearing the name "Elements of Geology," and which supplemented his great work called "Principles of Geology, " the publication of which at once gave its Author a position in the field of scientific research, and philosophical generalisation founded on research , which no future progress made in the same channels of human investigation is ever likely materially to modify. Of what materials then is the Earth composed? This evening I propose to invite the Members of the Victoria Institute to the consideration of some of the phenomena presented by one of those materials, and that one is the substance known to mineralogists by the name "Silex." This substance under a great variety of forms has a large share in the constitution of that small portion of the Earth beneath the surface accessible to human observation, and which, for the sake of convenience rather than correctness, is called its "crust." In this crust then we find as forms of Silex, the beautiful substance known as Rock-crystal, also jasper, Carnelian, Chalcedony, Agate, and many others; but the form of Silex with which everyone is familiar, and which in its mass exceeds by millions of times all other varieties of Silex put together, is flint, a material which in many parts of England is found so valuable in road-making and m building; many of the churches in East Anglia owing their high preservation and beauty to the flint stones so largely used in their construction. A geological student going into one of the numerous chalk quarries which are to be seen on both sides of the Thames between Gravesend and London, has his attention at once arrested by horizontal strata of flint stones imbedded in the chalk; these flint strata being separated by three or four feet of chalk. Attached to these flints and some times enclosed in them are various fossils of the same species as are found in the chalk, consequently the chalk and the flint, though so entirely distinct mineralogically, must be regarded as one geological formation. But flint does not characterise the entire thickness of the chalk, being found only throughout its upper portion. There its presence furnishes the geologist with both mineral and zoological evidence for the identification of the upper portion of the great chalk formation; and while on the one hand, mineralogists and chemists have occupied themselves in attempting to explain the solution of flint in an ocean which must have been so highly charged with lime, and its precipitation from time to time in the condition we now find it, paleontologists, attracted by the numerous organic bodies it preserves, have naturally been led to speculate upon what may be termed the cretaceous aspect of a mineral to which they owe the possession of some of the most interesting objects of their, study. Now flint is by no means peculiar to the chalk formation, but the conditions under which it comes under our notice in chalk, constitute a phenomenon of the highest possible scientific interest.