In the limestone was found the flint or chert used for weapons and instruments in early times.
Mines in Flint and in north Cardiganshire, which also yield a certain deposit of silver ore.
For the knives chipped from flint by prehistoric man see Archaeology and Flint Implements.
Thus Anglesea, Carnarvon, Merioneth and Flint were erected in North Wales; whilst out of the districts of Ystrad Tywi and Ceredigion in South Wales, the old dominions of the house of Dynevor, the counties of Carmarthen and Cardigan were formed.
Formerly flint and steel were everywhere employed for striking a light; and gun flints were required for fire-arms. A special industry in the shaping of gun flints long existed at Brandon in Suffolk.
The commonest aggregates are broken stone and natural flint gravel.
The prisms are necessarily compound, and usually consist of flint glass with compensating prisms of crown.
Experience shows that, although spherical pebbles are to be avoided, Portland cement adheres tightly to smooth flint surfaces, and that rough stones often give a less compact concrete than smooth ones on account of the difficulty of bedding them into the matrix when laying the concrete.
I. Flint Rocks; II. Clay Rocks; III. Lime Rocks.
Rutherfurd devised one made of flint glass with two crown glass compensating prisms; whilst Thallon employed a hollow prism containing carbon bisulphide also compensated by flint glass prisms. In direct vision spectroscopes the refracting prisms and slit are in the observing telescope.