OF CENTRAL CANADA PART III. 



183 



joints or partings in the direction of 

 which it separates more or less 

 readily, forming prisms or prismatic 

 masses of from three to eight or 

 nine sides : and as these possess also 

 transverse joints at right angles to 

 the axis of a prism, a flat, tabular, 

 and step-like outline is very gener- 

 ally presented by columnar or sub- FIG. 99. 

 columnar varieties of this kind. Hence the term "trap " or " trap- 

 pean rock," from trappa, a Swedish word signifying a set of steps 

 attention having first been called to this peculiarity by Swedish 

 observers. A good example is presented by the promontory of 

 Thunder Cape, Fig. 

 100, on the north- 

 west shore of Lake 

 Superior, in which 



five very distinct 



steps are observable, FIG. 100. 



more especially when viewed from a certain distance. The eruptive 

 mass of McKay's Mountain on the other side of Thunder Bay, as 

 well as similar rocks on Pie Island and elsewhere in that district, 

 exhibit also well-marked illustrations of this step-like outline, al- 

 though most of these rocks present only a sub-columnar structure. 

 A similar step-like outline is exhibited by some large dykes of col- 

 umnar dolerite in the township of Grenville on the Ottawa, as first 

 pointed out by the late Sir William Logan. 



As regards their general conditions of occurrence, greenstones and 

 traps are seen very commonly in the form of more or less broad and 

 straight or simply-forking veins (Fig. 101), technically known as 



dykes. This term originates in 

 the fact that trappean veins 

 usually possess greater powers 

 of resistance to the decompos- 

 ing influences of the atmos- 

 phere or the destructive action 

 of water than the rocks which 

 they traverse ; in conse- 

 01 quence of which they often 



