J. A. Dresser— A Rare Rock Type. 11 



Art. 'K.— On a Rare Rock Type from the Monteregian 

 Hills, Canada; by John A. Dresser. 



[Published by permission of the Director of the Geological Survey of 

 Canada.] 



The Monteregian Hills form a well-recognized petrographic 

 province* consisting of eight hills composed of igneous rocks 

 in the St. Lawrence valley extending along a line from Mount 

 Royal at the city of Montreal eastward for a distance of fifty 

 miles. They are a series of volcanic necks or laccoliths intru- 

 sive through Paleozoic sediments. The intrusions took place 

 probably in Devonian times, since which there has been a long 

 period of erosion succeeded by heavy glaciation, thus leaving 

 hills of the butte type and composed of plutonic rocks. They 

 are comparatively fresh and lend themselves particularly well 

 to the method of determination required by the Quantitative 

 Classification, which proves an invaluable aid in correlating 

 them. 



The rocks of these hills are those characteristic of.alkaiic 

 magmas and the province may be compared to that of Essex 

 county, Massachusetts, the Magnet Cove district, Arkansas, 

 the Crazy mountains of Montana, in the United States of 

 America, or to the Christiania district in southern Norway, or 

 the Kola peninsula, Finland. In each of the bills there is a 

 large development of essexite or theralite, and in all that have 

 been studied in detail an alkali syenite, pulaskite, nordmarkite 

 or nepheline syenite has been found. There is thus quite a 

 wide range of composition between the different rocks of the 

 individual hills. The mean composition of the hills compared 

 one with another also varies considerably, but this variation is 

 expressed in the different proportions of the essexite and 

 syenite groups rather than by the occurrence of widely different 

 rock types. The basic rocks are more extensively developed 

 towards the western end of the group. 



St. Bruno Mountain is the local name of the second of the 

 Monteregian Hills from the western end. It is fourteen miles 

 east of Montreal, near the line of the Grand Trunk Railway 

 between Montreal and Portland or Quebec. 



Many years ago a rock was noted from this hill by the late 

 T. S terry Huntf to which he gave the name of "olivinitic 

 dolerite or peridotite," and which is a somewhat different type 

 from any of the series yet described. Hunt observed that 

 olivine was the preponderating mineral in some portions of the 



* Adams, F. D., "The Monteregian Hills, a Canadian Petrographic 

 Province," Journal of Geology, vol. xl, No. 3. 

 f Geology of Canada, 1863, p. 665 et seq. 



