not, however, confined to Primordial rocks, but occur in altered sediments of various ages. The Primordial sediments must have at one time been very widely distributed, and must have filled up many of the inequalities produced by the rending and contortion of the Laurentian beds. Their thicker and more massive portions are, however, necessarily along the borders of the Laurentian continents, and as they in their turn were raised up into land, they became exposed to the denuding action first of the sea, and afterwards of the rain and rivers, and were so extensively wasted away that only in a few regions do large areas of them remain visible. That of Bohemia has afforded to Barrande a great number of most interesting fossils. The rocks of St. David's in Wales, those of Shropshire in England, and those of Wicklow in Ireland are also of great interest; and next to these in importance are, perhaps, the Huronian and Acadian groups of North America, in which continent--as for example in Nova Scotia and in some parts of New England--there are extensive areas of old metamorphic rocks whose age has not been determined by fossils, but which may belong to this period. The question of division lines of formations is one much agitated in the case of the Cambrian rocks. Whether certain beds are to be called Cambrian or Silurian has been a point greatly controverted; and the terms Primordial and Primordial Silurian have been used as means to avoid the raising of this difficulty. Many of our division lines in geology are arbitrary and conventional, and this may be the case with that between the Primordial and Silurian, the one age graduating into the other. There appears to be, however, the best reason to recognise a distinct Cambrian period, preceding the two great periods, those of the second and third faunas of Barrande, to which the term Silurian is usually applied. On the other hand, in so far as our knowledge extends at present, a strongly marked line of separation exists between the Laurentian and Primordial, the latter resting on the edges of the former, which seems then to have been as much altered as now. Still a break of this kind may be, perhaps must be, merely local; and may vary in amount. Thus, in some places we find rocks of Silurian and later ages resting directly on the Laurentian, without the intervention of the Primordial. In any case, where a line of coast is steadily sinking, each succeeding deposit will overlap that which went before; and this seems to have been the case with the Laurentian shore when the Primordial and Silurian were being deposited. Hence over large spaces the Primordial is absent, being probably buried up, except where exposed by denudation at the margin of the two formations.