GEOLOGICAL SOCIETY OF TENNSYLVANI A. 363 



the wall and the vein : small branches are also frequently 

 found to diverge from them on both sides. 



With all these descriptive particulars, however, it is 

 frequently very difiicnlt to distinguish true from contem- 

 poraneous veins. There are few veins which can be 

 brought to the test of all these marks ; and there are 

 probably exceptions to some of them. Some veins which 

 are very short, are perhaps true veins, and others of 

 considerable length and width, and tolerably straight, 

 may possibly be contemporaneous ; but with respect to 

 the latter, if the dip of the strata of the rocks in which 

 they are found be minutely examined, it will probably 

 appear that^, in most cases, the inclination of those veins 

 is parallel to it, and that they are therefore contempora- 

 neous rather as beds than veins. Other veins there are, 

 of such a dubious nature, in many respects, as to render 

 it necessary to place them in a different and separate 

 order. 



We shall briefly notice the various veins which would 

 seem to come under the denomination of contemporane- 

 ous veins, presenting such details as will illustrate the 

 peculiarities of each. 



Granite veins, in a granite formation, occur in various 

 parts of Cornwall, Great Britain. They are of three 

 kinds. Some are of considerable size, and their granite 

 is of the same kind and colour as that of the environing 

 rocks, but rather decomposed; they have no regular 

 walls; their direction is about north and south. The 

 second class comprises those which differ only in co- 

 lour from the granite which contains them ; these are 

 generally not more than one foot in width, and its large 

 crystals of feldspar are of a beautiful red colour, while 

 thoseof the adjoining granite are brownish-white. The 

 veins of the third class are composed of compact and 

 fine-grained granite, very different from the contigu- 

 ous granite. They occur from six inches to a foot in 



