EELATIOXS OF THE MESNAIM) QUAETZITE. 231 



soliists jiiid cneisscs. In rlic localities wIktc tlie liasal coiio-loineratcs occur 

 the proof of the uucouforuiable relations is conclusive. In other locali- 

 ties the s>ranite was apparently decomposed before the de])osition <il' the 

 quartzite, and here, as has Ix-en said, it yielih'd its small separate mineral 

 particles to the overhing- rock. This recomposed rt)ck has been tlior- 

 ou"-hh- cemented. Durino- the subsequent fidding- shearing has taken jdace 

 alone the junction, resulting in the development of parallel schistosity in 

 the original granite and in the recomposed rock. In such cases it is difficult 

 or impossil)le to indicate the exact contact between the Basement Complex 

 and the Mesnard (piai-tzite. Such localities were explained by llominger 

 as cases of progressive inetaniorphism, the granite being a metamorphosed 

 sedimentary rock. Later he abandoned this view. One of the best htcal- 

 ities in which to observe this apparent gradation between the gneissoid 

 granite and the quartz-schist is just north of the little granite knob on 

 whose south side is the west (piarter post of sec. 1, T. 47 N., R. 2") W. 

 (x\tlas Sheet XXXIX). This c(mtact between the Mesnard (piartzite and 

 the Archean atfbrds an excellent illustration of the princii)le that crys- 

 talline gneissoid granite may grade stej) by stej) into a rock which is an 

 unquestionable quartzite, there being no sharp line of demarcation between 

 the two, and vet between the two formations there really being a profound 

 nnconformity. 



THICKNESS. 



As the Mesnard quartzite is the first formation of a transgressing sea, 

 it doubtless originalh' varied in thickness, this l)eing due to irregularities 

 of the x\rchean basement. This irregularity in the basement is indicated 

 by the tact that the quartzite belt is in one place scarcely more than 150 

 feet across. As the dips are here vertical, this may be taken as the thick- 

 ness of the formation. From this thickness the quartzite shows a continuous 

 exposure at Mount Mesnard of 700 feet, which Avitli an inclination of <S0° 

 corresponds to a thickness of HTO feet. In other ])laces the belt is much 

 wider than this, but here the increased widtli is plainly due to folding, and 

 even at j\Iount Mesnard the iuterstratified belts of slate and gray wacke may 

 contain minor rolls which have escaped observation and the real thickness 

 of the formation be less than (50(1 feet 



