1 70 Geological History of Blackford Hill. [Sess. 



sequently formed, the fissures of the rock, indispensable to 

 the formation of vein agates, may be of any age from the 

 consolidation of the rock onwards ; and the deposition of the 

 vein material may have an indefinite range in time. It is 

 abundantly evident from specimens of Blackford Hill vein 

 agate that a vein complete at one time may be reinforced 

 by subsequent splitting and re -cementation along the re- 

 opened deposition plane. There are examples where a frac- 

 tured sheet of vein agate has split, first along one plane, then, 

 transgressing, along another, the new vein material consoli- 

 dating in this way. Again, a vein, after completion, may be 

 brecciated, and a subsequent vein formed, with the brecciated 

 fragments floating, as it were, in the later vein. At first 

 sight complex veins present a bewildering problem, so diffi- 

 cult is it to account for the fantastic mosaic found in such 

 veins. 



Familiarity with varying forms enables one at length to 

 classify them more or less satisfactorily into groups. The 

 simplest manifestation of a vein is a green film, Mr Good- 

 child's (1) "Priming," the initial skin upon which agates are 

 based, attributed to the decomposition of the ferro-magnesian 

 minerals, and identified by Dr Heddle as Celadonite ; (2) A 

 vein of jasper or clear chalcedony intercalated between films 

 of " priming " ; (3) Compound veins, exhibiting rectilinear 

 planes of alternating jasper and varieties of chalcedony ; (4) 

 Breccia veins, with fragments of the andesite embedded in 

 the vein ; (5) Brecciated veins, where the fragments of pre- 

 viously formed veins are embodied in the matrix of secondary 

 veins. To this group one might assign reinforced veins, but 

 if there is no apparent transgression by the reinforcing 

 material, the vein will apparently belong to the third group. 

 Besides these are faulted veins, some of which admirably 

 illustrate regional crustal movements. It is significant that 

 " normal " faults in the veins are rare, indicating lateral com- 

 pression as a feature in the vicissitudes of the rock. Many 

 veins occupy slip planes, showing well-marked slickensiding, 

 but the commoner examples of slip planes have calcite in 

 place of silica. Rarely veins exhibiting the characteristic 

 salient and re-entrant angles of fortification agates are found, 

 and occasionally a true fortification agate in a cavity formed 



