F. P. Mennell—Archcean Stratigraphy. 257 



of rock that we are accustomed to in England, we find miles of 

 almost continuous sections along hill-slopes and stream-beds. Even 

 a comparatively bare tract like the Alps is at a disadvantage owing 

 to its ups and downs, whereas the African tablelands afford almost 

 plane surfaces. The remarkable rocks that we used to puzzle over 

 through the microscope are clearly shown in their relations with the 

 other formations, and we are able to form definite conclusions where 

 before we could but guess. The writer has scarcely passed a day 

 for four years without setting foot on Archaean rocks. And he may 

 place on record his deliberate opinion that there is no greater rarity 

 than a " rock of doubtful origin," if we except, perhaps, a few 

 talcose rocks and others usually occurring around mineral deposits, 

 in whose formation hydrothermal agencies have played a great part. 

 There does not appear to be any problem of this nature that 

 combined field and microscopic observation is not competent to 

 solve. The coarsely crystalline schists which have excited so much 

 controversy are found to have been crystalline from the start, as 

 they appear to be invariably of igneous origin, for it can be proved 

 to demonstration in the majority of cases, and must be accepted 

 as an inevitable inference in the rest. Most gneisses are, in fact, 

 igneous rocks of quite recent date in comparison with the sediments 

 into which they can be seen intrusive. These sediments are rarely 

 of a highly crystalline nature, save along contacts, and show their 

 true characters both in their composition and their field relations. 

 Careful search indeed rarely fails to reveal exposures where they 

 are comparatively free from alteration, and show clearly their 

 sedimentary origin even in hand specimens. The groups which have 

 perhaps been the greatest puzzles in other parts of the world are 

 here quickly reduced to order. The banded gneisses, granulites, 

 etc., whose secret seemed so impenetrable when their field relations 

 were imperfectly understood, are no longer a mystery when we can 

 examine them along bare rock surfaces miles in extent. Some are 

 granites modified by movements in consolidation and the local 

 absorption of particular classes of rock ; others are schists modified 

 by contact action and impregnation with granitic material ; others, 

 again, can be termed neither igneous nor metamorphic, but may be 

 conveniently classed as ' mixed rocks,' having been formed by the 

 interlamination of igneous and other material due to the ' lit par 

 lit ' injection processes which characterise the plutonic masses when 

 we get near their roots. 



Among rocks such as we have alluded to, the principles of 

 ordinary stratigraphy have little application. Terms like dip and 

 strike become almost meaningless, and the structural relations of 

 the rocks have to be made out from quite other considerations than 

 superposition. The apparent dip is simply that of foliation, and 

 though the strike often coincides, as pointed out by Judd,^ with the 

 real direction of the strata, this is by no means always the case, as 

 may soon be found out by the simple experiment of following the 

 apparent strike of a band with well-marked lithological characters 



1 Student's Lyell, p. 549. 



DECADE T. — VOL. III. — NO. VI. 17 



