derived in this paper should be used. Graphic solutions are apt to introduce larger errors. If the Brunton compass is used to measure dips less than 10° it is unneccessary to use telescopic alidade and stadia to measure distances, for pacing and hand level have the same order of accuracy and the results will commonly have a probable error of 10 per cent. or more. 3. If, where stratigraphic units are bounded by beds that dip from 20° to 80° and the difference in dip is less than 10° a probable error of less than 5 per cent. is desired, measurements of distance and difference in elevation should be made by telescopic alidade and stadia; and measurements of dip by using the Brunton compass or similar clinometer. Thicknesses should be calculated by formulæ rather than graphic methods. If errors greater than 5 per cent. are permissible, graphic methods may be used. 4. If, as above, stratigraphic units are bounded by beds that dip from 20° to 80°, and the difference in dip is more than 10°, measurements should be made by telescopic alidade, stadia, and Brunton compass, and graphic methods based upon parallel folding will yield results with a probable error of 5 per cent. The formulas here set forth will probably save time, however, particularly if several calculations have to be made. Other conclusions may be drawn from the data given by those especially interested in the subject. The writer has used the formulas and methods set forth in this paper in the study of a highly folded region in Wyoming where several hundred sections ranging from 500 to 15,000 feet thick have been measured. Although it is recognized that the nature of the exposures, purpose of the work and time available for it will determine the order of accuracy obtainable in measuring stratigraphic sections, it is also firmly believed that the merits of precise methods justify their wider use. Certain problems in stratigraphy and in the structure of folded regions can only be advanced by close attention to accurate methods of measuring sections. U. S. GEOLOGICAL SURVEY, CHALCOPYRITE DEPOSITS IN NORTHERN MANITOBA. E. L. BRUCE. The deposits of chalcopyrite recently discovered and developed in Northern Manitoba lie near the western boundary of the province, a short distance northwest of Lake Athapapuskow, which is shown on Maps of Canada, fifty miles north of the Saskatchewan river. A large part of the journey from the railway at the town of The Pas, must be made by canoe in summer, or by sleigh in winter, and all the ore has been brought out during the winter months to the lakes that can be reached by steamers, and thence by scows down the Saskatchewan river. TOPOGRAPHY. The topography is rather monotonous for most of the distance from the railway to the producing area. Saskatchewan river flows in a number of changing channels through swampy country, and in many places is separated from broad, shallow lakes only by willow-covered levees. These lakes and the muskeg areas that border them act as reservoirs which are flooded during periods of high water. Northward from the Saskatchewan the same monotonously flat muskeg country continues to the boundary between the Pre-Cambrian and the Palæozoic formations. The rocks of the latter in many places stand up in an escarpment 70 to 80 feet above the lower lying but more rugged surface of the older rocks. In contrast to the unbroken level of the southern district with shallow, islandless lakes with low shores, the northern district abounds in ridges and hills, has only small muskeg areas and is dotted with almost countless, clear, island-studded, rock-bounded lakes of all sizes. GENERAL GEOLOGY. Since the rocks associated with the ores are all of Pre-Cambrian age it will be necessary only to mention that Palæozoic dolomites cover the older rocks south of lake Athapapuskow, and that Glacial and post-Glacial deposits form a discontinuous cover of varying thickness over all of the consolidated rocks. The Pre-Cambrian succession is as follows: Granite Granite-gneiss Hybrid granitic rocks Intrusive contact Arkose and conglomerate Unconformity Slate Unconformity? Granite porphyry Intrusive contact Sedimentary and igneous gneisses Volcanic Rocks.-The oldest rocks are comparable to the oldest rocks of Pre-Cambrian age found in other areas, but, as they are separated from similar formations by great stretches of granites and gneisses to the east and by the Paleozoic cover to the south, it is not and may never be certain that they are of the same age as the Keewatin rocks of Lake Superior. They consist almost entirely of volcanic rocks of various types. The most common variety is massive, green to grayish-green in color and weathers to brownish color. The ellipsoidal structure figured in so many descriptions of Pre-Cambrian lavas is very strikingly developed and there are, in addition, certain other typical volcanic forms. Autoclastic bands have been formed by the rolling of the semi-solidified lava and by shearing. Pyroclastic beds are common. They have been formed by the incorporation of bombs of all sizes, from those a foot in diameter down to the finest ash, in the still molten rock. The pyroclastics have, superficially, much the appearance of some of the altered conglom erates of the district. All types, but chiefly the massive lava without the ellipsoidal structure, have been changed, locally, into chlorite schist by pressure and shearing. Some of this alteration occurred before the first period of erosion since, in the conglomerates overlying the volcanics, there are pebbles of schist which have not been sufficiently deformed by movements affecting the conglomerate to account for their schistose character. Associated with the volcanics there was probably deposited a certain amount of normal sediments, now altered to schists indistinguishable from those of igneous origin. Complex of Gneissic Rocks.-Outside of the district in which the sulphide ores have been found there are areas of hornblendic gneisses and garnetiferous schists and gneisses that represent in part a sedimentary series apparently overlying the volcanics but with no recognized unconformity. These gneisses are intruded by granite sills and are cut by innumerable pegmatite dikes. The sills and dikes are believed to be of different ages, some of them older, some younger, than the conglomerates and arkose that lie above the greenstone series. Granite Porphyry.-There is some evidence to show that one small batholith of granite porphyry is older than at least part of the sediments. The rock has rather peculiar bluish phenocrysts of quartz and under the microscope shows unusual, irregular, graphic intergrowths of quartz and feldspar, similar to intergrowths that occur in granite pebbles from the conglomerate. Slate. A narrow belt of slate has been found bounded on both sides by greenstone schist. No unconformity has been noted between the two rocks but the slate is lithologically like that lying unconformably above the volcanics farther west. Conglomerate and Arkose.—The first formation that is clearly unconformable above the volcanic complex in the area is a thick series of conglomerate, arkose, and greywacke, now lying in closely folded synclines or as infaulted blocks in the older rocks. The folding following the deposition of these sediments has been so intense that even pebbles of quartz, granite, and quartzite, have been twisted, bent and drag-folded. Some of the lower beds consist largely of debris from the greenstone complex and these have been reconstituted into rocks so closely resembling the igneous original that it is hard to delimit the exact boundaries of the sedimentary synclines. Bedding in the sediments is not plainly marked, but in many places there is a lenticular arrangement, with very sudden variations both in composition and texture. These sudden changes, as well as the mineral constituents of the rocks, show rather conclusively that this series was deposited as outwash fans by rapid streams and probably under arid conditions. The close folding and the faulting that the sediments have undergone make a measurement of the thickness impossible but it must have been thousands of feet. Granite Gneiss and Granite.-The diastrophic period that followed this final Pre-Cambrian sedimentation was accompanied by intrusions of granite. These took place through a considerable length of time and the intrusives vary somewhat in character but are believed to be differentiates from a common magma rather than products of different periods of intrusion. The ore is thought to be genetically related to certain of these differentiated magmas. Texturally, the granitic rocks are massive granite, in places porphyritic, and granite gneisses, faintly banded owing to a concentration of the basic constituents biotite and hornblende. There is no evidence of any great secondary rearrangement as the longer axes of the biotite foils are not parallel to the banding. In composition the gneisses are practically identical with the massive granite. |