THE 



JOURNAL OF GEOLOGY 



JANUARY- FEBRUARY, 1893. 



ON THE PRE-CAMBRIAN ROCKS OF 

 THE BRITISH ISLES. 



During the last twenty years much has been written about 

 the " pre-Cambrian " rocks of the British Isles. Unfortunately 

 when attention began to be sedulously given to the study of 

 these ancient formations, the problems of metamorphism were 

 still a hundred fold more obscure than they have since become ; 

 the aid of the microscope had not been seriously and system- 

 atically adopted for the investigation of the crystalline schists^ 

 and geologists generally were still under the belief that the 

 broad structure of these schists could be treated like those of 

 the sedimentary rocks, and be determined by rapid traverses of 

 the ground. We have now painfully discovered that these older 

 methods of observation were extremely crude, and that the work 

 performed in accordance with them is now of little interest or 

 value save as a historical warning to future generations of geol- 

 ogists. Geological literature has meanwhile been burdened with 

 numerous contributions which remain as a permanent incubus on 

 our library shelves. 



It may serve a useful purpose at the present time in possibly 

 aiding those who are engaged in the study of the oldest rocks 

 of North America, if I place before them, as briefly as possible, 

 the main facts which in my opinion have now been satisfactorily 

 proved regarding the corresponding rocks of Britain, and if I 

 indicate at the same time some of the more probable inferences 

 in those cases where the facts, at present known, do not warrant 

 a definite conclusion. 



A'oL. I, No. I. 



