Petrographical Relations of Laurentian Limestones. 9 



ambiguity or uncertainty, they are denned at this point 

 in the exact sense in which they are used here. Foliation 

 is a "laminated structure, produced in a rock by the 

 parallel arrangement of certain or all of its constituent 

 minerals " ; handing is the alternation in the form of 

 bands, of gneisses differing more or less in composition or 

 structure, which gneisses may or may not be foliated as 

 well ; granitic is the typical structure of granite, charac- 

 terized by a general lack of crystalline form and a more 

 or less complete interlocking on the part of the mineral 

 grains; granular is the structure common to granites 

 which have been somewhat deformed and have in con- 

 sequence lost much of this interlocking of the grains ; 

 graniditic structure causes a thin section of a rock possess- 

 ing it to appear as a mosaic of roughly equidimensional 

 grains, usually of small size — it is typical of recrystallized 

 rocks which have been subjected to movement during the 

 process of solidification, but it may be produced by the 

 deformation and crushing of already solidified granitoid 

 rock-masses. 



The rocks of this area, with the exception of the true 

 limestones, present a similarity in one respect, namely, 

 they are all foliated ; in general, also, they are banded. 

 Otherwise, however, as has already been mentioned, they 

 present a wide variation in character, ranging in composi- 

 tion and likewise in properties from acid granites to basic 

 amphibolites. 



Perhaps one of the most noticeable features in the 

 petrography of the more acid types is the occurrence of 

 plagioclase feldspar, often in great preponderance over 

 the orthoclase ; a rock which is to all appearances a granite 

 when examined macroscopically, being possibly of a pink 

 color; and highly quartzose, is found under the microscope 

 to be really a quartz diorite. This phenomenon, known 



