TYPICAL LAURENTIAN AREA OF CANADA. 333 



feldspars, as distinguished from " Orthose," or orthoclase feldspar, 

 and thus equivalent to the term plagioclase now in general use, 

 but having no connection with anorthite, a variety of plagioclase 

 which is seldom present. After a careful study of, these rocks, 

 both in the field and the laboratory, it is believed that this name 

 should be retained for this well-marked member of the gabbro 

 family, which, though not a common rock elsewhere, has an 

 enormous distribution in the Laurentian of Canada. 



If an olivine gabbro be regarded as the central member, so 

 to speak, of the gabbro family, the replacement of the mono- 

 clinic by rhombic pyroxene will give rise to an olivine norite. 

 A gradual diminution in the amount of plagioclase will give rise 

 to a peridotite or gabbro pyroxenite, a diminution in the amount 

 of pyroxene to a troktolite or plagioclase-olivine rock, while a 

 diminution in the amount of olivine and pyroxene will give rise 

 to an anorthosite, which variety forms the greater part of the 

 intrusive masses in question. The gradual passage of one variety 

 into another can be distinctly traced in many localities in the 

 anorthosite masses. These anorthosites are in some places mas- 

 sive, but very frequently show a distinct foliation, often very 

 perfect. In some places they occur interbanded with the gneiss 

 and crystalline limestone, while elsewhere they cut directly 

 across the strike of these rocks. The interbanded anorthosite, 

 together with the gneiss and limestone associated with it, was 

 supposed by Logan to form a distinct sedimentary series, to 

 which the name "Upper Laurentian," or " Norian," was given, 

 because the discovery that elsewhere the anorthosite runs across 

 the strike of the gneiss was supposed to indicate that this series 

 covered up and unconformably overlay the Grenville series, the 

 igneous and intrusive character of the anorthosite not being 

 recognized on account of its frequently foliated structure. It is 

 now known that these anorthosites do not constitute an inde- 

 pendent formation, but are igneous rocks which occur, cutting 

 both the Grenville series and the Fundamental gneiss. They 

 have, however, in many cases been intruded before the cessation 

 of the great dynamic movements to which the Laurentian was 



