Slate is, as we know, merely a variety of argillite. Slate quarries are found in England, Switzerland and Italy, but it is in France especially that the industry has been most extensively developed by reason of the large deposits that underlie its surface, particularly in the province of Anjou, where they extend from Trelaze to Avrille, a distance of six miles, and in the department of Ardennes, at Remogne, Fumay, etc.
Normandy, Brittany, Dauphiny and Marne likewise possess quarries, although they are not so productive.
The exploitation is commonly done in open quarry. After the vegetable mould (which in this case is called "cover") has been removed, we meet with a solid slate which it is difficult to split into laminæ, and it is not until a depth of at least fifteen feet is reached that we find a material that is fit to be exploited. All the best beds of slate, in fact, improve in quality in proportion as they lie deeper under the surface, near to which they have little value. Without entering into details as to the exploitation of this product, let us say that the blocks have to be divided in the quarry, since, in the open air, they rapidly lose the property of readily splitting into thin, even laminæ.
[Illustration: SLATE STORE-VATS FOR BREWERIES.]
Slate has but slight affinity for water, and, moreover, resists atmospheric influences, humidity and heat pretty well.
This property renders it valuable for a large number of domestic purposes.
There is no certain proof, it is true, that it was employed by the ancients, but it is, nevertheless, extremely probable that it was used in mass at an early period for stair heads, pillars for buildings and as a material for fencing.
The exploitation of the material became especially active at the period when the idea occurred to some one to use slate for the rooting of houses. It was employed for this purpose along with tiles as far back as the eleventh century in the majority of schistose districts. It is well known, for example, that Fumay (Ardennes) at this period had a brotherhood of slate quarrymen.
A method of getting out the material and cutting it regularly was found toward the end of the twelfth century, and it was not till then that it became of general application. Moreover, with the advent of the Gothic period slate became indispensable for castle roofs, which have a conical form.