346 GEOLOGICAL APPEARANCES 



same ; and that so far the main rocks and the veins are of 

 the same formation, or have a common origin. This infer- 

 ence will probably be disputed by some, and I shall therefore 

 offer a few remarks in support of it. 



116. All would admit, that every part of one of the main 

 rocks of sienite in the bed of the Tilt was of the same forma- 

 tion. As in any other case of a rock of a crystallised aggre- 

 gate, such as granite, greenstone, &c. the continuity of the 

 mass, and its uniform, or gently varied, characters, are the 

 grounds for the inference. It is the most simple hypothesis, 

 and the rule that " No more causes are to be admitted than 

 " are sufficient to account for the phenomena," authorises us 

 to conclude in favour of one productive process, where no rea- 

 sons can be shown for inferring more than one. We have 

 seen, however, that among the veins there are some con- 

 sisting of a sienite precisely similar in its characters to that 

 of the main rocks ; and that some such veins are con- 

 nected with the main rocks, without any abrupt transition. 

 Now, where both these circumstances occur in the vein, the 

 vein and the main rock are parts of one continuous and ho- 

 mogeneous mass ; and every reason for admitting that any two 

 parts of the main rock are of one formation, goes equally to 

 prove that the main rock and the vein are of one formation, 

 except in what regards the difference of form in the vein. 

 But this difference affords no just ground of distinction. In 

 several places, especially about the bridge, there are separate 

 masses of the strata imbedded in the sienite. When such 

 masses lie far asunder, the sienite between them is regarded as 

 a part of the main rock : if we look out for such as have still 

 smaller and smaller intervals between them, we shall by and 

 by find the intervening sienite to obtain the form, and the 

 name of a vein ; and the sooner, according as we select larger 



masses 



