PRIMARY AND SECONDARY STRESSES 



RECORDED BY THE VEIN SYSTEMS IN THE PERCE ROCK 



BY JOHN M. CLARKE 



(Plates 30-32) 



The Perce rock in Gaspe, P. Que., to which reference has fre- 

 quently been made in these publications, is a block of lower 

 Devonian limestones standing almost on end. An isolated mass out 

 of the flank of a great anticline of Appalachian type, it is a part of 

 the mountain folds of the Gaspe peninsula. The strains to which it 

 has been subjected through upturning of its strata were naturally 

 manifold and as some part of these were torsional the rock mass 

 has been considerably cross-checked by fissures now filled out of 

 calcite veins. 



There is, however, a very evident record of successive strains 

 which, when closely read, gives an interesting clue to the history 

 of the deformations; and this record is illustrated in plan by the 

 accompanying sketch made by the writer. Doubtless there are 

 like evidences of other disruptions which may point to later stages 

 in the development of the rock mass. . 



In studying the figure on plate 30 the observer is looking 

 down on the vertical strike edges of the strata, which are made up 

 of limestones largely composed of fine sand grains stained brilliantly 

 by red and yellow iron oxides, the former, a bright brick red, 

 predominating. Of the two vein series crossing these strata, the 

 older is an anastomosing network in which the main strands have a 

 generally parallel direction. These veins are all solid white calcite, 

 filling the fissures entirely and streaked lengthwise by black 

 deposits of impurities. The veins of this series are of prevailingly 

 larger size than the others and may be several feet across. Gener- 

 ally they contain no inclusions of wall rock or other material and 

 are thoroughly healed wounds of the rock mass. 



The secondary series crosses the primary at a bold angle ; in 

 one place, however, a branch of this fissure series has opened 

 along the middle of one of the primary veins. These veins are 

 red in general cast of color, brighter even than the mother rock ; 

 carry well-defined wallbands of crystalline calcite and later bands 

 upon these ; they are filled with a breccia of small horses which are 



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