'What may be termed,' we quote from the _Testimony of the Rocks_, 'the three geologic days--the third, fifth, and sixth--may be held to have extended over those Carboniferous periods during which the great plants were created--over those Oolitic and Cretaceous periods during which the great sea-monsters and birds were created--and over those Tertiary periods during which the great terrestrial mammals were created. For the intervening, or fourth day, we have that wide space represented by the Permian and Triassic periods, which, less conspicuous in their floras than the periods that went immediately before, and less conspicuous in their faunas than the periods that came immediately after, were marked by the decline and ultimate extinction of the Palæozoic forms, and the first partially developed beginnings of the secondary ones. And for the first and second days there remains the great Azoic period, during which the immensely developed gneisses, mica-schists, and primary clay-slates were deposited, and the two extended periods represented by the Silurian and Old Red Sandstone system. These, taken together, exhaust the geological scale, and may be named in their order as, first, the Azoic day or period; second, the Silurian, or Old Red Sandstone day, or period; third, the Carboniferous day, or period; fourth, the Permian or Triassic day, or period; and sixth, the Tertiary day, or period.'