REPORT ON MINERAL VEINS. 13 



He remarks, however, as to the veins of Cornwall, that their 

 great irregularity in size and in form, their fi'equent ramifica- 

 tions, their similarity of composition and intimate connexion 

 with the rocks which they traverse, and, above all, the large 

 masses of slate which they envelop, are all circumstances to 

 disprove their origin from fissures, and to support their con- 

 temporaneous origin. 



Dr. Boase suggests that veins follow the arrangement of the 

 joints of the rocks, and that it may thus be explained why the 

 different series of veins cross each other, and why the veins of 

 each series are respectively parallel. 



And he thinks that thus we may suppose how veins which 

 are crossed may seem to abut or terminate against those that 

 are opposed thereto ; having, when in the same line, that pecu- 

 liar appearance that has been attributed to intersection, and 

 the appearance of being heaved when on the opposite sides of 

 the cross vein, they are not on the same line, but occur in the 

 parallel joints of distant layers. 



The latter occurrence, he remarks, although very common, 

 is not however universal ; for, in some instances, the part of the 

 vein supposed to have been intersected has never been found. 



As Mr. Carne had observed, that when contemporaneous 

 veins meet each other in a ci'oss direction, they do not exhibit 

 the heaves and interruptions of true veins, bvit usually unite. 



Dr. Boase says that this statement is opposed to his obser- 

 vations, and that the phaenomenon of intersection is common to 

 all kinds of veins. Further, he expresses a doubt whether 

 heaves in veins are not after all rather apparent than real, but 

 explains that he does not mean to assert that they do not ex- 

 hibit these phaenomena, but that this arrangement, as in the 

 case of small veins, only gives the appearance of being moved 

 from the original positions. 



I have now stated the opinions which, as far as I know, have 

 been generally received on the subject of the formation of veins, 

 from which it wiU appear that there are three leading hypo- 

 theses. 



1st. That which supposes them to have been open fissures, 

 caused by disruption, and occasioned principally by subsidence 

 of parts of the rocks, which fissures were afterwards filled up 

 with various matters by deposits from aqueous solution, chiefly 

 from above. 



Modifications of this theory are. That such rents in the earth 

 may have been caused in other ways, such as earthquakes, or 

 certain great convulsions, as well as by subsidence : 



That they may have been filled by the infiltration of sola- 



