GEOLOGY OF OGDEXSBURG 1 3 



these intrusives the name of Laurentian is appUed; for the younger 

 set certain of the Canadian geologists have recently suggested the 

 name '' Algoman." It is not certain to which of these groups the 

 granites in Dekalb and in Macomb belong. But both granites 

 and syenites are well represented in the small Precambrian area 

 which the maps contain. 



Much later in Precambrian time came renewed igneous intrusion, 

 and black, heavy lavas, so-called trap rocks, rose toward the sur- 

 face. Such traps are well represented in the Thousand Islands, but 

 in the district here mapped but two sm.all dikes of this rock have 

 been seen. 



The Grenville rocks in considerable diversity are found in the 

 southern part of the Ogdensburg quadrangle. They are cut by 

 granitic, syenitic and gabbroic eruptives, and by trap dikes. 



The Precambrian rocks exposed within the mapped area cover 

 such a trifling amount of territory, being the mere northern fringe 

 of the great and well-exposed areas of these rocks on the Gouver- 

 neur and Hammond sheets next south, that it seem_s unwise to 

 attempt any elaborate investigation of them until these sheets have 

 also been studied. Since it is our present plan to commence study 

 of the Gouverneur area, the detailed report upon the Ogdensburg 

 Precambrian will be left until that work is completed. 



Grenville series. The Grenville series in the Adirondacks 

 exhibits an enormous but unknown thickness of limestones, quart- 

 zites and various sorts of schists and gneisses. ' These are water 

 deposited rocks and were imquestionably deposited in great thick- 

 ness over the entire region. Not long after their deposit, however, 

 they were invaded from beneath by huge masses of molten granite ; 

 and at a subsequent time by even greater masses of a variety of 

 igneous rocks. This action broke up the old series into a group 

 of disconnected blocks and patches, separated by masses of the 

 intrusives, so heated and compressed the sediments as to cause 

 complete recrystallization of their constituent parts, and vastly 

 changed their appearance. The pure limestones were changed into 

 white marbles, with scales of yellow mica and of graphite always 

 present. The less pure limestones are full of other silicates in 

 addition, chiefly pyroxenes and scapolite. The sandstones were 

 changed into quartzites and quartz schists. The shales were altered 

 to schists of various sorts. Contact rocks were produced at the 

 contacts of many of the igneous rocks, and these also have been 

 recrystallized to schists and gneisses. Often there is great difficulty 



