The siliceous limestone, which contains also a burr-stone, covers half the bottom of the Paris basin. In the middle and upper beds of the calcaire grossier they have hornstone, and the nectic and lenticular quartz. The singular substance, called silex menilites, is well known to all mineralogists: it is found in the foliated argillaceous marls of the gypsums. In these marls they have also siliceous nodules, which are white, opake, flat, and mammillated, and also beds of flint. Silicified trunks of trees are also described. The meuliere without shells, above the upper marine formation, passes often into the state of flint, sometimes white and opake, and sometimes grey and translucent.
In their upper freshwater formation, a siliceous part containing the same shells as the calcareous is very abundant. The description of the several varieties of flints in these beds is as follows.
- Flint, of a grey colour, translucent, fracture close and waxy, and even horny.
- Flint, yellow, very translucent, very easy to break, fracture conchoidal and smooth.
- A jasper flint of an opake whiteness like wax, fracture waxy and scaly, possessing little frangibility.
- An opake jasper flint, a little cellular, and having all the characters of a compact burr-stone.
Such is the imperfection of language, that a correspondence merely in the general description of two mineral substances, is not sufficient to enable us to ascertain their identity. It must be allowed however, that most of the characters above enumerated will apply very accurately to many of the flints which I have described as composing the greatest part of some heaps of our gravel. I have also found among these many water-worn fragments of a siliceous stone that bears a considerable resemblance to the French meuliere or burr-stone.