r78 NEW YORK STATE MUSEUM 



cutting the syenite lead to the impression that it is later than 

 that, but there is some question as to the correctness of their 

 reference to the main gabbro of the region. There may have been 

 two periods of gabbro outflow, one earlier and the other later 

 than the syenite. 



A few dikes, and a few small eruptive masses, of three or four 

 different types have been noted which are not referable to any of 

 the great masses apparently. But as yet their relationships are 

 obscure. All are younger than the anorthosite, and all are 

 metamorpho'sed. 



The last outflows from the Adirondack eruptive center con- 

 sisted of diabase and of syenite porphyry, both found only in the 

 dike form. That they are considerably younger than any of the 

 foregoing is evidenced by their wholly unmetamorphosed condi- 

 tion. They are however pre-Potsdam and in all probability pre- 

 Cambrian. The diabases are exceedingly close to the gabbros in 

 composition. The syenite porphyries are higher in the alkalis 

 and lower in lime the magnesia than the syenites. They have 

 a wide range in composition but are mostly quite acid and 

 quartzose, belonging to the nordmarkite type of Brogger. In 

 one case the diabase has been shown to be the younger. The 

 only considerable differences between the gabbro and diabase 

 are to be seen in the high titanium of the former and the high 

 magnesia of the latter. The syenite porphyries depart con- 

 siderably from the syenite, as has been already pointed out. 

 They have a very localized distribution in the Adirondacks as 

 compared with the diabases, and similar dikes have not been 

 noted, so far as the writer is aware, in Norway or in Canada 

 associated with the diabase. 



12 3 4 5 6 



Si02 47.42 46.73 63.45 63.02 68.5 68.96 



AI2O3 17.34 16.66 18.38 14.87 14.69 15.25 



FesOs 4.91 3.56 .42 6.53 1.34 3.28 



FeO 10.22 8.45 3.56 none 3.25 none 



MgO 5-21 8.12 .35 .95 .26 .2 



CaO 8.09 8.03 3.06 1.12 2.2 .76 



Na20 3.48 3.73 5.06 5.85 3.5 5.45 



K2O 1.89 1.64 5.15 5.62 5.9 5.01 



H2O 1.13 2.39 .3 1.45 .4 .91 



