REPORT OF TEE CHIEF ASTROXOMER 101 



SESSIONAL PAPER No. 25a 



MacDonald Formation. 



Above the Hefty formation in the conformable Galton series is a thick 

 division of beds which betoken the long continued deposition of rather uniform 

 sediment. Since these rocks underlie an extensive surface in the MacDonald 

 range, they have been grouped under the name of the MacDonald formation. 

 Good exposures are found among the ridges overlooking the Wigwam river. 

 The whole thickness was, however, not observed in any section traversed within 

 the Boundary belt. The estimated total, 2,350 feet, is doubtless not quite accurate 

 but the error is believed not to be great. 



The formation is notably homogeneous, so far as the main lithological 

 features are concerned. It was found in the field that a subdivision into three 

 members could be recognized with advantage. This subdivision is based largely 

 on differences in the colours of weathering, rather than on any fundamental 

 differences of composition or origin. The lowest member, 550 feet thick, 

 weathers characteristically light brown or brownish gray; the middle member, 

 1,100 feet thick, weathers light gray, and the top member, 700 feet thick, 

 weathers light brown or buff, though a few beds weather gray. 



The principal rock type throughout the formation is a highly silicious 

 argillite or metargillite, quite similar to the standard phase of the Appekunny. 

 The colour of the fresh rock is a light gray, or more commonly, light greenish- 

 gray. The bedding is usually thin but becomes more massive in the lowest 

 member of the formation. Sim-cracks and ripple-marks, especially the former, 

 are abundant in many horizons ranging from summit to base. The top member 

 carries some thin intercalations of red, sandy, or argillaceous strata, indicating 

 a transition to the overlying Wigwam formation. Along with these there occur 

 a few dolomitic lenses, rarely over six inches thick, which bear a number of 

 flattened concretions. The concretions range up to a foot in greatest diameter. 

 They are composed of alternating concentric layers of different carbonates. 

 The bulk of each concretion appears to be a ferruginous dolomite forming 

 layers from one-eighth to one-quarter inch in thickness. These are separated 

 by much thinner laminae of nearly pure calcite which, on the weathering of the 

 rock, is dissolved away at a more rapid rate than the dolomite. At first sight 

 the weathered section of one of the concretions suggests Cryptozoon or other 

 possibly organic form, but the writer believes that the organic appearance is 

 deceptive and that the structure is due to a physical and chemical rearrange- 

 ment in magnesian limestone, analogous to that causing the molar-tooth struc- 

 ture of the Siyeh limestone. 



About 500 feet above the base of the middle member a thin (two-inch) 

 but remarkably persistent layer of gray (brown-weathering) somewhat mag- 

 nesian oolite was seen in several traverses separated by distances of from two 

 to eight miles. The spherical grains have the usual concentric and radial 

 structure and average about 1 mm. in diameter. They are cemented by calcite 

 and infiltrated quartz. Some of the quartz present is probably of clastic origin 

 as it is associated with grains of microcline. A variable proportion of a grain 



