1S67.] MACPARLANE — GEOLOGY OP LAKE SUPERIOR. 189 



of the brecciated rocks above described differs in composition from 

 the fragments which it encloses, we nevertheless find that the two 

 are usually so intimately combined with each Qther as to behave 

 under the hammer like one and the same rock. There is, in the 

 majority of cases, no joint to be found at their junction with each 

 other; and in fracturing them, they very often break just as 

 readily across as along the line which separates them. It would 

 appear, therefore, that, although these rocks solidified at different 

 times, the dates of their formation were not sufficiently far 

 distant from each other to enable the previously existing rock to 

 cool thoroughly before it became penetrated by or enclosed in the 

 newer one ; that consequently the older rock, being in an intensely 

 heated condition, readily amalgamated at its edges with the next 

 erupted and fused mass, and formed with it a solid compact whole. 

 Apart from the difficulties which would doubtless attend any 

 attempt to distinguish separate geological groups among these 

 rocks, it would appear just as unreasonable so to separate them, as 

 to regard each distinct stratum of sedimentary rock as distinct 

 geological formations. According to Naumann, a geological 

 formation consists of a series of widely extended or very numerous 

 rocks or rock-members (Gehirgs-gliedcr), which form an indepen- 

 dent whole, and are by their lithological and palseontological 

 characters, as well as by their structure and stratigraphical suc- 

 cession (Lagerungs folge), recognisable as contemporaneous (geo- 

 logically speaking) products of similar natural processes. According 

 even to this definition, it would appear just to class all the rocks 

 above described, in spite of the distinctly intrusive character of 

 some of them, as belonging to one and the same geological forma- 

 tion, — in short, to the Laurentian series of Sir W. E. Logan, 

 or the Primitive Gneiss formation of Naumann. The last-named 

 geologist certainly distinguishes a separate granite formation, but 

 the rocks included in it are generally more recent than the primi- 

 tive gneiss or primitive schists. Where, as in Silesia, in Podolia on 

 the Dnieper, in the central plateau of France, in Finland, in Scan- 

 dinavia, and in the Western Islands of Scotland, granite occurs 

 in similar intimate association with gneissoid rocks as on Lake 

 Superior, Naumann always regards it as part and portion of the 

 primitive gneiss. As early as 1826, Hisinger, in his work on 

 Swedish mineralogy, shewed that the granite which occurs in 

 intimate combination, by lithological transition and otherwise, with 

 the primitive gneiss of Scandinavia, was of contemporaneous origin 



