506 J. A. DRESSER IGNEOUS ROCKS OF EASTERN QUEBEC 



ancient tuff beds, or at least are composed of fragmental volcanic ma- 

 terial, and so are pyroclastic rocks. In the present degree of alteration 

 they do not differ essentially from certain portions of basic rocks just 

 described. Besides, these are siliceous rocks, quartzites, graywackes, and 

 chloritic sandstones, which are possibly true sediments. With them are 

 frequent beds of dolomite, the origin of which seems a matter of doubt. 

 The rock is frequently found resting on a basic trap, filling pit-holes and 

 interstices within it and inclosing fragments of it. In other cases the 

 rock passes by rather gradual transition into quartzose dolomite in masses 

 of considerable extent. At the Eustis mine the portion of the country 

 rock known to the miners as the "green rock" is of this type. Even in 

 thin-sections small areas of dolomite appear, sometimes inclosing small 

 quartz crystals and indicating the secondary nature of the dolomite. 



Certain of the micaceous chloritic slates also contain sufficient amount 

 of dolomite to cause a slight effervescence with hydrochloric acid when 

 heated. 



Along the Saint Erancis river the Sutton belt is some 7 miles in its 

 extreme width, including nearly 2 miles of recognized Trenton measures 

 within it. A detailed study shows the volcanics at the base with dolo- 

 mite, quartzite, and gray mica-chist, in ascending order. Within the 

 dolomite are certain peculiar inclusions of a bluish gray limestone which 

 has been largely crystallized by intense regional metamorphism. One of 

 these inclusions, however, contains fossil evidence of its Calciferous-Chazy 

 age. It is therefore demonstrated that in this part at least the Sutton 

 Mountain belt contains no pre-Cambrian elastics. 



The following is the section referred to crossing the pre-Cambrian near 

 Saint Erancis river. 



The line of section* extends between lots 10 and 27, in range 12, in the 

 township of Cleveland, and is about a mile and a half east of Saint Fran- 

 cis river and approximately parallel to it. The direction of the section 

 is north 38 degrees west, magnetic. The adjacent rocks at both north 

 and south have been recently mapped in the reports of the Geological 

 Survey as Cambrian, the black limestone number 3 of the section as 

 Trenton, and the rest as pre-Cambrian. 



* This locality was considered by Logan to furnish the key to the structure of the 

 Quebec group. Hither he returned after severing his connection with the Geological 

 Survey, and spent four seasons in making a detailed map of the district for several 

 miles on either side of this section. This map, which seems to have been ready for 

 engraving at the time of his death, was unfortunately never published. 



