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The Rocks Of a"Kaaa;--.aK::aa.L aaaa; THE ROCKS OF '•_: a 'i":aai •••*«>»••'>»>*»< DEPOSITED BY THE COMMITTEE OX (Braouate Stuoies. 11 11 r-~. ' W«-^-»* ..I.......*».*'" ix1 . \p\A «\t)0^> ACC NO. DATE A P E T R 0 G R A P H I C ^ L STUDY OF THE E 0 G K S OF IS 0 U N T ROYAL. 1. INTRODUCTION. 11. LITERATURE ON THE SUBJECT. 111. PETROGRAPHY OF THE ESSEXITE. IV. PETROGRAPHY OF THE NEPHELINP SYENITE. V. A PETROGRAPHICAL STUDY OF THE DYKE ROOKS. 1. INTRODUCTION. Mount Royal, situated as it is on the south-east border of the Island of Montreal, is surrounded for many miles by an almost flat plain; this plain is bounded on the north by the foothills of the Laurentian Plateau and extends southward to the Appalachian Mountains; this almost flat plain was once the bottom of an early Palaeozoic sea, and wfc**jk now constitutes the broad valley of the River St. Lawrence. Mount Royal is the most westerly of a group of hills which cut up thru these palaeozoic sediments - and which constitute the group known as the Monteregian Hills(? this group of hills gives us the best known example of a Petrographical Province in Canada. Standing alone, as Mount Royal does, on the westerly border of this ancient sea-bottom, it serves to break the monotony of the landscape and can be seen from many miles distant in any direction. This mountain represents the base or root of a once active volcano , but has been extinct for a long geological period of time; the top of this ancient volcano stood much higher than we see it today but owing to erosion and denudation it has been reduced Dr.F.D.AdaraB -"Thejtfnteregian Hills "journal oc? Geol. jl. 1902 -2- reduced to its present altitude which is about 750 feet above the St. Lawrence River,or 769.6 feet above sea level. The area occupied by Mount Royal (proper) is about one and a half square miles-r and is somewhat oval in shape, the narrower end pointing southwards. The mass of the mountain is composed of igneous rocks which together with the surrounding strata are much out up by a numerous and complicated series of dykes, and it is a petrographical study of thesevarious ig­ neous rooks and more especially of the dykes, with which this paper will deal. The rooks which compose the igneous mass of Mount Royal are Essexite, and Nepheline Syenite, and the dyke rocks which are varied in their mineralogical composition. As stated before Mount Royal is the remains of an ancient volcano in which three distinct periods of eruption are clearly recognized; the first of these eruptions gave rise to the Essexite, forming the eastern portion and occupying about seven-eights of the present mountain. A second disturbance tapped the reservoir and allowed the magma to flow up on the west side of the Essexite, this time as an acid magma, crystallizing out as Nepheline Syenite, now seen as a narrow band on the northwest flank of the mountain. Some time after this magma had become entirely solidified -3- there occured a third disturbance, probably not so violent as either of the preceeding ones but cer­ tainly much prolonged, which cracked not only the former intruded igneous masses, but also shattered the surrounding strata for many miles and filled up many of these fissures with material which we now study as dyke rock. As has been stated these dykes are found cutting Syenite both the Essexite, the Nepheline^and the surrounding sediments, and as well traversing one another, which shows that the earlier intrusions had sufficient time to solidify before they were traversed by the later dykes. The majority of these dyke rooks are much more basic than either of the former intrusions, yet it seems quite probable that all three igneous masses have come from the same magma. The sequence of eruptions, which show a dif­ ferentiation of the magma, is interesting as well as worthy of notice; it affirms the hypothesis made by Ton Richthofen regarding the succession of acid and basic magmas; that is to say, a magma of medium ba$i|ity being intruded first, is succeeded by other intrusions at either extremes. Such is the succession in the oase of Mount Royal, the medium basic Essexite constituting the -4- the first intrusion, is followed by an acid variety, seen in the Nepheline Syenite, and constituting the second intrusion; this in turn is followed by dykes more trachytic in character at first and finally by the very basic dykes and sills. The dykes may be regarded as vertical or in­ clined walls of igneous rock, which are closely as­ sociated with the rocks of two preceeding eruptions. Besides these vertical dykes, there are several sheets or sills of the same rock material intercalated between the beds of limestone. These dykes and sheets represent the closing stage of volcanic activity in Mount Royal; many of these can be seen at various places about the moun­ tain, more especially in quarries, such as Mile End Quarries, Corporation Quarry - at Outremont, or Testmount Quarry, as well as on St. Helen's Island, Mount Royal Park, Fletcher's Field and Back River, The writer has made a careful microscopic study of several of these dyke rocks, from the following localities - the bed of the River St. Lawrence between St. Helen's Island and the Yictoria Bridge which dykes are at most timer covered by water; St. Helen's Island; the Reservoir Extension - also covered by water; Mile End Quarries; Maisonneuve; Back "River; and as well the slopes of the mountain. A description has been -5- written about each of these dyke rocks - and these have been catalogued. Specimens of a great many of these dyke rocks have been collected at various times and by several people-; these specimens have been stored in the Petrographical Labratory of McGill University, await­ ing detailed examination. As a great number of these dyke rocks are now covered up and unavailable, the material in the Petrographical Labratory is the only record which we have of many of these dyke rocks. Besides making a special study of the dyke rocks, the writer, in this paper, has noted the results that have been obtained by others, regarding the Essexite and Nepheline Syenite, and has verified many of these, by means of the microscope. A comparison has been made of these, with similar occurences. Finally, there has been considerable literature written on the rocks of Mount Royal, by various writers, at different times ; these papers have been reviewed and a resume made of the results obtained by the various writers. 11. LITERATURE ON THE SUBJECT. The earliest literature on Mount Royal and vicinity was that written by Sir Wra. Logan and Dr. Sterry Hunt in the - Geology of Canada - and published in 1863 by the Geological Survey of Canada, in which is given a general description of the different sedimentary strata and the areas occupied by each, as well as a crude description of its igneous intrusives, dykes and inter­ calated sheets, vhich are classed as trachyte ,phonolite, diorite and dolerite; the last two represent the Essexite mass and certain dykes. This report also contains a considerable number of chemical analysis of certain rocks and more especial­ ly of certain constituent minerals in the various rocks,- these analyses being determined chiefly by Hunt. Many rocks, more especially the dyke rocks are described by these men, but most of these are so ^ne and often imperfectly described, that it is ver-y dif­ ficult sometines to recognize the specific rook describ­ ed; but this must necessarily be expected as the use of the microscope, and modern petrographic methods were unknown to them. During the next thirty years after the publication of the Geology of Canada - onlyfpapers appeared, dealing in part with Mount Royal. The first of these was R -V- reference made by Dr. B. J. Harrington on the oo- •f curence of sodalite.natrolite and analcite in cer- ''•> *• tain dyke rocks from the Reservoir on the slope of the mountain;- this appeared in -"Notes on a Few Canadian Minerals and Rocks" published in Annual Report of the Geological Survey of Canada 1874-75 Pg. 302. Another paper by the same writer appeared three years later in the Report 1877-78 pg. 42 G - which was called - On Some of the Diorites of Montreal - in which the writer described certain typical diorite dyke rooks, from the vicinity of Mount Royal, together with an analysis of one of these; he also refers to the age of the eruptive rocks. In 1888 M. A. Lacroix visited Montreal and after returning home to France he published his pamphlet - "Description des Syenites Nephdlinigues de Pouzao (Hautes - PyrSn^es et de Montreal (Canada) et de leurs phenomena de contact." - Bull. Soc. G6ol. de France, 3e s^rie, *ome X^lll, 1890. In this paper Lacroix discusses the rocks com­ posing Mount Royal and especially the Nepheline Syenite; T* he also describes a similar occurence of Nepheline Syenite at Pouzao - T^jxoneeB Mountains - which locality was later visited by Dr. F. D. Adams in connection with -8- the Eighth International Geological Congress - and which he vividly describes in the - Journal of Geology- Yol. IX pg. 28 - 1901. This paper by Lacroix, with the exception of his study of the Nepheline Syenites of Mount Royal, is largely a compilation of results of previous ^bors by other men. He describes the rooks under three groups - (1) Diabases and Teschenites, which cor­ respond to our Essexite.
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