324 



Sketch of the Geology of 



echellon movement of the trap. How then shall we 

 explain the mode in which this appearance was pro- 

 duced ? I can conceive of only one explariation, and 

 that not free from difficulties. All will now admit, I 

 suppose, that all trap rock had an igneous origin* Let 

 us now imagine that the melted mass, at this spot, was 

 forcing its way upwards between the layers of the slate. 

 It is not difficult to conceive that the opening, towards 

 the extremities, might pass between other layers of the 

 slate than those which separated along the central parts ; 

 an occurrence which one might see produced by endeavor- 

 ing with a wedge to separate a mass of slate rock. The 

 opening would be widest where the wedge entered and 

 become gradually narrow towards the extremities, where 

 would probably be seen the scaling up of different layers. 

 In this way, portions of the melted trap rock might be 

 lodged between different layers of the slate, without any 

 apparent fracture of the intervening layers, when they 

 were inspected only on their bassetiing edges. Yet were 

 this the mode of its intrusion, if the trap were uncovered, 

 its apparently disconnected portions would be found to 

 unite at no great depth. 



In the gneiss formation northwest of Portland, we find 

 frequent examples of greenstone veins, from one to five 

 feet wide. In some places the gneiss (here losing all 

 marks of a stratified or slaty structure, and being a real 

 granite in the immediate vicinity of the trap,) is so divided 

 by the ramifications of the greenstone veins, that one can 

 hardly say whether it be veins of trap In gneiss, or of 



gneiss 



be broken off, containing two 



can 



or three very distinct 



alternations of the two rocks. So firmly are they united, 

 that they separate with little less facility at their junction, 



^ 



