GEOLOGY. 



By T. STERRY HUXT, LL.D., F.R.S., 



Professoe of Geology, Institute of Technology, Boston, Mass. 



PEE-CAMBRIAN ROCKS. 



The attention of geologists is more and more drawn to the 

 study of the stratified crystalline rocks. So long as these 

 were supposed to be due to the alteration, over certain re- 

 gions, of sediments of Paleozoic or more recent times, they 

 possessed but a secondary interest ; but since we have learn- 

 ed to recognize in them portions of great series of higher an- 

 tiquity, they assume a new importance to the student, alike 

 from the geognostic and the geogenic side. As regards the 

 former, much progress has been made in their lithology, pa- 

 leontology, and stratigraphy; while the question of their ori- 

 gin brings up in new forms the old questions of plutonism 

 and neptunism. The microscope is now applied with great 

 success to the study of crystalline rocks, but the opinions of 

 microscopists on many points of lithology are as yet unset- 

 tled, and the criteria upon which some have relied to distin- 

 guish between eruptive and indigenous rocks are found to 

 be fallacious ; while the tendency of the latest results, in the 

 judgment of many, is towards a limitation of plutonism and 

 a wider extension of neptunism. 



WALES. 



The crystalline rocks of Wales, noticed in the Record of 

 last year as pre-Cambrian, have been further examined with 

 important results. The groups to which Hicks gave the 

 names of Dimetian and Pebidian were first found in a ridire 

 protruding from the Cambrian strata at St. David's, in Pem- 

 brokeshire, South Wales, but have since been found in a sec- 

 ond parallel ridge about ten miles to the east, on which is 

 situated Castle Roche, and also in North Wales, both in 

 Caernarvonshire and Amrlesea; in each one of the four dis- 

 tricts presenting the same geognostical and lithological re- 



