These divisions were not of the same length. The Palæozoic ages were probably far, far longer than those of any other division, while the Quaternary period is shorter than any of those which preceded it. Each of these main divisions is divided further into systems or shorter periods (just as the dynasties of ancient Egypt could be subdivided into reigns). Though the broad outlines of the sequence of the living things which existed in those periods has been the same all over the world, many local differences may be traced in the nature and grouping of the sedimentary materials in which the remains of the living things of these epochs have been preserved. To find the oldest rocks, we must seek those which lie at the bottom or underneath all the others. Judged by this test, the oldest rocks in Great Britain are certain hard rocks (like gneiss, or the material of which volcanic veins are composed) which crop out in the north-west of Scotland, and which form the outer Hebrides. They are also known in Anglesea, and in the extreme west of Wales, at St. David's. Similar strata form the Malvern Hills of Worcestershire, the Longmynd Hills, Caer Caradoc and the Wrekin Hills of Shropshire, and the hilly district of Charnwood Forest in Leicestershire. For a long time the Cambrian rocks of Wales, so called from North Wales's ancient name of Cambria, were believed to be the oldest on the face of the earth. Up to the year 1830 even these rocks had no name or recognition, for geologists believed that it was impossible to classify them. But in 1831 Professor Adam Sedgwick, of Cambridge, began the diligent study of the rocks in North Wales, and after five years' work he was able to announce in 1836 that he had determined the general order of succession in that district of a certain ancient group of slaty, gritty, and flaggy strata. However, eighteen years later, in 1854, Sir William Logan, who was then engaged in mapping the rocks of Canada, found along the River St. Lawrence an enormous thickness (30,000 feet or more) of gneiss, quartzite, schist,[15] limestone, etc., these rocks underlying--and being, therefore, older than--the Cambrian strata, which are also well developed in that country. To these "bottom" rocks Logan gave the name of Laurentian. For some time afterwards the same name was also applied to the somewhat similar rocks which were found to underlie the Cambrian formation in Britain, but it was felt safer to give the English rocks a more general name. They are therefore now usually called Pre-Cambrian, which simply means older than the Cambrian strata, or Archæan.