THE ADIRONDACK (1UAIMI 1TK DEPOSITS 123 



THE IGNEOUS ROCKS 



The Algoman. The anorthosite, syenite, granite and gabbro 

 (arranged in order of their age), which compose the younger group 

 of igneous rocks, the writer regards as Algoman in age. The)' have 

 been so thoroughly discussed in the various bulletins of the New 

 York State Museum as to call for no further discussion. The 

 writer would take more kindly toward the view that the syenite- 

 granite masses represent several different centers of intrusion con- 

 temporaneous in age rather than that they represent differentiated 

 portions of a single body. 



The name Algoman, perhaps, needs a word in the way of explana- 

 tion. Correlation of igneous rocks is, at the best, difficult, but the 

 Precambrian rocks of Canada have been studied in sufficient detail 

 to furnish data for numerous correlation tables, twenty of which 

 have been examined. There is a striking similarity in nearly all ; 

 there are only two periods of igneous activity prior to the 

 Keweenawan. The other granite of the Adirondacks, Cushing 

 regards as Laurentian. Hence, if we follow Miller and Knight, 1 

 we are compelled to employ the term Algoman. It furnishes a 

 much desired " handle." 



The metagabbro. This igneous rock found on a number of the 

 graphite properties has not received the attention in Adirondack- 

 geology it deserves. The cause of this is not difficult to find. It 

 has been pointed out that the amphibolites are often difficult to 

 interpret as they have been derived in a number of ways: (i) 

 Grenville para-amphibolites, (2) metamorphosed basic (femic) 

 igneous rocks such as gabbros, diorites, and diabases, (3) contact 

 metamorphosed impure limestones. 2 Excluding the last as unim- 

 portant in the present discussion, the choice rests between para- 

 amphibolites and ortho-amphibolites. Cushing, and especially 

 Martin, 3 lean strongly toward the igneous interpretation. While it 

 is true that some of the amphibolites are igneous, it is not necessarily 

 the case in all. 4 Granting that some are sedimentary and some are 

 igneous, a distinction between the two is difficult and frequently 

 impossible, for they are often confoundingly alike. 



Specimens were collected from rock masses where field relations 

 pointed to a definite origin. Microscopic examination revealed 



1 Miller, W. G., & Knight, C. W., Jour. Geol., 23:588. 



2 Cushing, H. P., N. Y. State Mus. Bui. 191, p. 15. 



3 Martin, J. C, N. Y. State Mus. Bui. 185, p. 57. 



4 Lowinson-Lessing, F., " Uber die chemische Natur der'feldspath Amphibole. 

 Ann. de l'lnst. Polytech. Pierre le Grand. St Petersbourg, 15:559-76, 191 1. 



