REVIEWS 417 



types of the series are intimately interbanded and in many cases one type 

 gradually grades into the other. The dark-colored bands and spots 

 rich in silicate minerals, in the crystalline limestone, are thought to 

 represent recrystallized sedimentary matter and not to be due to the 

 introduction of silica, iron, or alumina from the gabbroic or granitic 

 magmas. Although some of the metamorphic amphibolite is of igneous 

 origin and younger than the Grenville sediments, most of the field 

 evidence points to a sedimentary or contact metamorphic origin for the 

 garnet gneiss and a large part of the amphibolite. The different varieties 

 of garnet gneiss and amphibolite associated with the igneous contacts 

 probably are due to the different phases of intensity in the contact action. 



The post-Grenville pre-Cambrian rocks are gabbro-amphibolite and 

 granite gneiss intrusives. Two noncomformable contacts and their 

 mineralogical composition indicate that certain of the amphibolites are 

 igneous. The abundant inclusions of Grenville sediments in these 

 amphibolites indicate their post-Grenville age. These gabbro- 

 amphibolites are intruded by a gray to pinkish red massive to gneissoid 

 granite gneiss. In almost all cases, owing to the severe regional defor- 

 mation, the original nature of the contacts between the various types of 

 pre-Cambrian rocks of this region is obhterated and it is very hard to 

 work out field relationships. Inclusions of gabbro-amphibolites in 

 pegmatitic phases of the granite gneiss prove a later age for the granite. 

 Also during the intrusion of the granite magma, the amphibolite inclu- 

 sions broke under deformation while granite flowed, which shows that 

 the amphibolite antedates the intrusion of the magma. However, in 

 only a very few cases is there evidence that these amphibolite inclusions 

 are igneous and not metamorphic Grenville sediments. The great 

 abundance of these inclusions, and the apparent incompetency of granite 

 contact alteration to account for so much, suggests that much of it was 

 formed by the gabbro-amphibolite intrusions before the invasion of the 

 granite magma. 



The rocks are almost universally foliated and over the whole area 

 the fohation dips toward the west or northwest. A huge sigmoid fold 

 in the southern part of the area is described and diagrammed in detail. 

 Many minor folds are described. 



This is an interesting description and apphcation of a number of 

 important principles of pre-Cambrian geology, especially the methods of 

 separating the sedimentary and contact metamorphic amphibolite from 

 the igneous gabbro-amphibolite, and the determination of the younger 

 age for the granite as compared with the gabbro-amphibolite. 



J. F. W. 



