RELATIVE DATES OF INTRUSIVE ROCKS. 107 



renewed efforts, discovered and made fully known. While rendering 

 a just homage to those who have made plain hitherto untrodden ])aths, 

 let it not be forgotten how much remains to be known, how far we are. 

 after all that has been aceomplislied, from a full and perfect know- 

 ledge of the infinite wonders of the created world. Our philosophers 

 of the loftiest intellects and of the largest attainments, need not, like 

 Alexander, mourn that there is nothing left for them to conquer. 



EELATIVE DATES OF VAEIOUS ITS'TEUSIVE ROCKS 

 CUTTIN& THE LAUEENTIAN SEEIES IN CANADA. 



BT SIR W. E. LOGAN, T. E. 



^ead hefore the Canadian Institute, December 12tJi, 1857. 



In describing the distribution of the crystalline limestone bands 

 of the Laurentian series of rocks, in previous Geological papers, it 

 became necessary to allude to a large area of intrusive syenite. 

 There are other igneous rocks, however, in the same district, and the 

 relative dates of all that were observed are very well marked. 



The oldest intrusive masses are a set of greenstone dykes, com- 

 posed of a greenish white feldspar and black horrblende, Avith a 

 small amoimt uf iron pyrites. Their width varies from ten to one 

 hundred yards, and they all possess a well marked transverse 

 columnar structure. The largest are occasionally moderately coarse- 

 grained, and the smaller fine-grained, but they are all distinctly 

 crystalline. Their general bearing is east and west, but the main 

 dykes occasionally divide, a branch striking off" at an angle of from 

 twenty to forty degrees. 



Tiiese greenstone dykes being always cut off by the syenite where 

 they have been observed to come in contact, it is plain the syenite 

 must be of posterior date. The area which the syenite occupies has 

 •^een elsewhere described. In its lithological character the rock is 

 very unifoinn, being composed for the most part of feldspar, either 

 of some tinge of red or a dull white, Avith black hornblende, and a 

 rather sparing quantity of translucent quartz. The red tinge pre- 

 vails more on the west side, the white on the east. In the spur 



