484 Canadian Record of Science. 



throughout these counties. They do not cut the strata 

 of Cambrian and Silurian age, which are found at a 

 number of places overlying the Laurentian, nor are any 

 surface flows known to occur, although, from the charac- 

 ters of one set of very fine-grained porphyrite dykes which 

 will be described, it seems not unlikely that such flows 

 have taken place over the Laurentian series, but the mate- 

 rial of which they were composed has been removed by the 

 excessive denudation to which these rocks have been 

 subjected since pre-Cambrian times. 



The late Mr. H. G. Vennor mentions a dyke, composed 

 of " a fine-grained, black, glittering dolorite, weathering 

 greyish-white," discovered by him near the banks of the 

 Kideau canal, in the township of North Burgess, and 

 states that " in width it varies from four to one hundred 

 feet." 1 Series of specimens were taken from this dyke, 

 and when examined in thin sections under the microscope 

 the rock is seen to consist of lime-soda feldspar and a 

 secondary ferro-magnesian mineral, together with small 

 amounts of brown mica, hornblende, and quartz. The 

 plagioclase occurs in more than one generation, so that 

 the rock is a porphyrite. The larger phenocrysts of this 

 mineral, whose length rarely exceeds 2*5 mm., occur 

 sparingly, and are of earlier formation than any of the 

 other essential constituents of the rock. The most 

 important ferro-magnesian constituent is a uralitized 

 pyroxene which occurs in irregular grains, enclosed, to 

 some extent, in crystals of the second generation of pla- 

 gioclase. Individuals of this latter mineral, whose average 

 length is under 0*35 mm., have, like the older phenocrysts, 

 the " dusted " appearance commonly seen in the feldspar 

 of gabbros. Some of the inclusions are air cavities. 

 Filling up the interstices between the two generations of 

 crystals mentioned are grains of quartz and feldspar, 

 some of which is probably orthoclase. A considerable 



1 Report of Progress, Geological Survey of Canada, 1872-3. 



