[Footnote 15: Hard rocks are sometimes composed of different minerals, which are arranged in a way that reminds us of a bed of fallen leaves, and are called "foliated," from the Latin word _folium_, a leaf. Gneiss is a good example of a foliated rock. It is composed of the three minerals, quartz, felspar, and mica, arranged in this foliated manner. Mica schist, talc schist, and other rocks have a similar structure, and are sometimes briefly called "schists."] In Canada the total thickness of the Laurentian, Pre-Cambrian, or Archæan rocks is now estimated at 50,000 feet. In Britain it is nothing like so great as this (though still considerable); but the thickness of these extremely old and altered rocks is a very difficult matter to determine, for all signs of the original stratification in them have often been destroyed, and the rocks have been so bent and folded that it is possible the same beds may have been measured more than once in the same section. It will be understood from some of the foregoing sentences that the task of dating or classifying these early rocks is one which is far from simple, and which has given rise to many different opinions. We may here give another example. "During the years between 1831 and while Sedgwick was occupied in studying the rocks of North Wales," writes Mr. W. Jerome Harrison, "another geologist, Mr. (afterwards Sir) Roderick Murchison, was engaged in the examination of the strata which occupy the south-east of Wales and the adjoining border counties of England. To these rocks Murchison gave, in 1835, the name of Silurian, from the ancient British tribe of the Silures, who inhabited that part of the country when the Romans invaded Britain." Later in last century, in order to distinguish more clearly the periods of the rocks which began or ended in these areas, the name of another ancient British tribe was called into requisition--the Ordovics; and thus for certain strata which were neither Silurian nor Cambrian Professor Lapworth proposed the name Ordovician.