ON THE CHEMISTRY OF METAMORPHIC ROCKS. 195 



Art. XVI. — On the Chemical and Mineralogical Relations of 

 Metamorphic Rocks ;* by T. Sterry Hunt, M.A., F.R.S. ; 

 of the Geological Survey of Canada. 



At a time not very remote in the history of geology, when all 

 crystalline stratified rocks were included under the commou 

 designation of primitive, and were supposed to belong to a period 

 anterior to the fossiliferous formations, the lithologist confined his 

 studies to descriptions of the various species of rocks, without 

 reference to their stratigraphical or geological distribution. But 

 with the progress of geological science, a new problem is present- 

 ed to his investigation. While palaeontology has shown that the 

 fossils of each formation furnish a guide to its age and stratigraph- 

 ical position, it has been found that sedimentary strata of all ages, 

 up to the tertiary inclusive, may undergo such changes as to 

 obliterate the direct evidences of organic life; and to give to the 

 sediments the mineralogical characters once assigned to primitive 

 rocks. The question here arises, whether in the absence of organic 

 remains, or of stratigraphical evidence, there exists any means of 

 determining, even approximately, the geological age of a given 

 series of crystalline stratified rocks; — in other words, whether the 

 chemical conditions which have presided over the formation of 

 sedimentary rocks, have so far varied in the course of ages, as to 

 impress upon these rocks marked chemical and mineralogical 

 differences. In the case of unaltered sediments it would be difficult 

 to arrive at any solution of this question without greatly multiplied 

 analyses ; but in the same rocks, when altered, the crystalline 

 minerals which are formed, being definite in their composition, 

 and varying with the chemical constitution of the sediments, may 

 perhaps to a certain extent, become to the geologist what organic 

 remains are in the unaltered rocks, a guide to the geological age 

 and succession. 



It was while engaged in the investigation of metamorphic rocks 

 of various ages in North America, that this problem suggested 

 itself; and I have endeavoured from chemical considerations, con- 

 joined with multiplied observations, to attempt its solution. In the 

 American Journal of Science for 1858, and in the Quarterly 

 Journal of the Geological Society of London for 1859 (p. 488), will 

 be found the germs of the ideas on this subject, which I shall 

 endeavour to explain in the present paper. It cannot be doubted 



* Read before the Dublin Geological Society April 10, and reprinted 

 from advance sheets of the Dublin Quarterly Journal for July 1863. 



