Ch. XXXII.] LAURENTIAN VOLCANIC ROCKS. 701 



water, and by the waste of such islands some of the ashy matter be- 

 came waterwom, whence the ashy conglomerate. Viscous matter 

 seems also to have been shot into the air as volcanic bombs, which 

 fell among the dust and broken crystals (that often form the ashes) 

 before perfect cooling and consolidation had taken place." * 



Laurentian Volcanic Hocks. — The Laurentian rocks in Canada, 

 especially in Ottawa and Argenteuil, are the oldest intrusive masses 

 yet known. They form a set of dikes of a fine-grained dark green- 

 stone or dolerite, composed of felspar and pyroxene, with occasional 

 scales of mica and grains of pyrites. Their width varies from a few 

 feet to a hundred yards, and they have a columnar structure, the 

 columns being truly at right angles to the plane of the dike. Some 

 of the dikes send off branches. These dolerites are cut through by 

 intrusive syenite, and this syenite, in its turn, is again cut and 

 penetrated by felspar porphyry, the base of which consists of petro- 

 silex, or a mixture of orthoclase and quartz. All these trap rocks 

 appear to be of Laurentian date, for the lowest fossiliferous rocks, 

 such as the Cambrian or Potsdam sandstone, overlie eroded portions 

 of them.f "Whether some of the various conformable crystalline 

 rocks of the Laurentian series, such as the coarse-grained granitoid 

 and porphyritic varieties of gneiss, exhibiting scarcely any signs of 

 stratification, some of the serpentines, may not also be of volcanic 

 origin, is a point very difficult to determine in a region which has 

 undergone so much metamorphic action. 



* Geol. Quart. Journ., vol. ix. p. 170, 1853. 

 f Logan, Geology of Canada, 1862. 



