254 PROCEEDINGS OF THE AMERICAN ACADEMY 



to which a proximately definite age can be assigned. They were pre- 

 ceded by very extensive eruptions of acid rocks, especially porphyries.* 



These basaltic rocks have been subjected to a wide-reaching altera- 

 tion, which has produced marked changes in the internal condition of 

 the beds, and has filled the fissures with a rich variety of minerals, 

 whose constituents were derived from the products of this alteration. 



In a previous article,t I sketched the relative ages of the minerals, 

 especially those occurring in the veins. 



In the present paper, I shall try to trace the changes that have taken 

 place in the interior of the rock masses, in places where the only 

 ino-ress and egress was through tlie capillary cracks formed by the 

 cleavage and mutual boundary planes of tlie crystalline constituents. 



The results at which I arrive are based on the study of several 

 hundred thin sections, made by myself in order that I might study 

 them before covering. The method followed was in general as 

 follows : — 



I. Examination of freshly broken surface under the microscope. 



II. Measurement of the angles of crystals, generally cleavage 

 angles, directly under the microscope, or with larger individuals on a 

 reflecting goniometer. 



III. Examination in detail of uncovered thin sections, trying hard- 

 ness with the point of a delicate needle (which was also magnetized 

 to detect magnetite) under any desired power up to a 4-10 objective. 



IV. Etching the uncovered section, first with acetic acid to detect 

 carbonates, then with muriatic acid. 



V. Study of the covered section under the microscope, in trans- 

 mitted and reflected light, and in polarized light. 



VI. In many instances, qualitative chemical determinations, by my 

 assistant, Mr. B. T. Putnam, were both necessary and decisive. 



VII. A small series of analyses was made for me, by Mr. George 

 W. Hawes and Mr. R. W. Woodward, in the laboratory of the Shef- 

 field Scientific School. 



* For recent discussions of the relatire ages of the Silurian sandstone, the 

 copper-bearing series, and tlie Huronian, tlie reader is referred to articles by 

 Brooks and Puinpelly (Amer. Jour. Sci., vol. iii., 1874, p. 428) ; R. Irving ( Amer. 

 Jour. Sci., vol. viii., 1874, p. 4G) ; T. B. Brooks (Amer. Jour. Sci., vol. xi., 1875, 

 p. 210) ; and Beports of the Geological Survey of Canada. 



t Paragenesis and Derivation of Copper and its Associates on L. Superior, 

 Amer. Jour, of Sci., 3d ser., ii., 1871, p. 188; also, Geol. Survey of Mich., 

 vol. i., part ii. 



