GEOLOGY OF SARATOGA SPRINGS AND VICINITY 1 7 



granite and gabbro, in the order named. These probably repre- 

 sent a group of closely related intrusions not widely separated in 

 time from one another. Much later came renewed igneous activity 

 with intrusion of diabase (trap), found exclusively as dikes cutting 

 all the other Precambric rocks. 



Representatives of all these rock groups occur in the Saratoga 

 region. The Grenville rocks are abundantly represented ; Laurentian 

 granite is probably present, that is to say there is abundance of a 

 granitic rock which we regard as probably Laurentian; the second 

 group of intrusions is represented by abundant syenite and ap- 

 parently by that alone; and occasional trap dikes of large size 

 belong to the last group. 



GRENVILLE SERIES 



The Grenville series in the Adirondacks exhibits an enormous 

 thickness of limestones, quartzites and various sorts of schists. 

 On the Saratoga quadrangle the bulk of the Grenville consists of 

 schists, but there is also a considerable amount of quartzite. Lime- 

 stone is present only as occasional thin bands in the quartzite 

 series. A few miles farther north, however, much more lime- 

 stone comes in. A considerable belt in which there is much quartz- 

 ite, interbedded with thin bands of schist and limestone, con- 

 trasts so strongly with the remainder of the Grenville that we have 

 mapped it separately. With this exception the intricate admixture 

 of various schists absolutely dehes detailed mapping. 



In addition, the series is everywhere cut to pieces by a white, 

 granitic rock of somewhat peculiar type, that we have heretofore 

 been regarding as a Grenville sediment. The reasons for regarding 

 it as an igneous rock will shortly appear. We have mapped sepa- 

 rately three areas of this rock, but the mapping is vague and highly 

 conventional. The rock is found everywhere throughout the Gren- 

 ville area, inextricably mingled with the schists. In these three 

 areas it exceeds the schists in quantity and is mapped separately 

 to give conventional expression to our views respecting its nature 

 and relationships. We are provisionally regarding it as Lauren- 

 tian, that being the term applied to the ancient granitic rocks which, 

 throughout Canada, the L^pper Lake region and the Adirondacks, 

 invade and cut to pieces the oldest known clastic deposits. The 

 uncertain feature of this correlation, so far as the Adirondacks 

 is concerned, is that the Laurentian granite, in the Lake Superior 

 region, is older than the Lower Iluronian. hence the use of the 



