440 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



which he sub-divided into two chief groups: the younger or 

 Algonkian (a term introduced by Walcott from the name of an 

 Indian race) comprises the clastic or crystalline rocks imme- 

 diately below the Cambrian system, while the older or Archcean 

 group comprises the wholly crystalline and foliated basement 

 complex of rocks. 



The pre-Cambrian phyllites, schists, and conglomerates of 

 Brittany have been subjected to a close examination by 

 Barrois (1883-94), who has greatly advanced the knowledge of 

 the stratigraphy of that complicated area. Barrois discovered 

 organic remains in the quartzites, and Cayeux identified them 

 as Radiolaria and sponges. Rauff, on the other hand, 

 regards these supposed fossil remains merely as mineral 

 structures. 



Alpine geologists have, in the course of detailed geological 

 surveys, frequently been able to prove that gneissose and 

 schistose areas of rocks which used to be regarded as pre- 

 Cambrian represent metamorphosed portions of the younger 

 formations. Even Cairiozoic rocks have undergone complete 

 metamorphism in highly-disturbed Alpine regions. 



The insuperable difficulty with which geologists have to 

 contend in their attempts to unravel the complicated strati- 

 graphical relations of metamorphic rocks is that, in virtue of the 

 changes they have undergone, any fossil remains which might 

 originally have been contained in them have been nearly all 

 altered beyond sure recognition. Then there is the other 

 difficulty that not only the sedimentary series, but also the 

 plutonic igneous masses and injected igneous rocks, when 

 they undergo strong crust pressures, may be converted into 

 foliated metamorphic rocks. Hence the only means of arriving 

 at a just appreciation of the age and relationships of the meta- 

 morphic rocks is, first, by careful cartographical survey and 

 comparison of the stratigraphical relations subsisting between 

 the several members of a metamorphic series and the sedi- 

 mentary unaltered rocks; and second, by finer microscopic 

 investigation of rock-specimens, taken from all grades of 

 altered and unaltered rocks whose relations in the field have 

 been fully investigated. 



Only after prolonged researches can geology hope to deter- 

 mine how much of the crystalline metamorphic rocks really 

 belongs to an Archaean and pre-Cambrian basement series, and 

 how much is of later sedimentary or igneous origin. 



