FAMILIES OF ROCKS 



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 dili- 

 gent study of the rocks in North Wales, and after five 

 years 1 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, 1 limestone, etc., these rocks underlying 

 and being, therefore, older than the Cambrian strata, 



1 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 



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