284 SMYTH ROCKS OF NORTHWESTERN ADIRONDACK REGION. 



quantity as to be of little importance in itself, while the fact that it cuts 

 all the other rocks renders it useless as a factor in working out their 

 relations. In microsections it usually shows fairly perfect ophitic struc- 

 ture, with the augite in a granulitic condition, instead of in large masses. 

 The only uncommon mineralogic features are the occasional presence of 

 garnet, which seems to be an original constituent, and the occurence of 

 some hypersthene. When the latter is in considerable amount and, as 

 is not uncommon, the ophitic structure disappears, there is a close resem- 

 blance between the diabase and the hypersthene-gabbro above described. 

 This fact, combined with their field characters, suggests a close connec- 

 tion between the two rocks. 



Summary. 



The region considered contains extensive belts of crystalline limestones 

 with interbedded gneisses, constituting a series which can hardly be re- 

 garded as of other than sedimentary origin. Certain hornblendic and 

 pyroxenic gneisses constantly associated with the limestone may be in 

 part modified igneous rocks. 



There are present several varieties of undoubted igneous rocks, granite 

 and gabbro in large amount, with minor diorite and diabase. They are 

 all younger than, and intrusive in, the limestone series, producing marked 

 contact-metamorphism in the latter. The ages of these intrusions with 

 reference to each other are not clearly determined except in the case of 

 the diabase, which is evidently the youngest. 



Besides these rocks whose origin is clear, there are wide areas of gneisses 

 whose origin and relation to the other formations are largely a matter of 

 doubt. Portions of these gneisses form the basal parts of the limestone 

 series. Other portions are intrusive, as shown by the granitic boundary 

 of one of the belts and by the passage of gabbro into a gneissoid form. 



Whether all of the gneisses belong in the one or the other of these 

 classes, or whether still other portions are older than, and unconformable 

 beneath, the limestone series, it is as yet impossible to say. At present 

 we have no absolute knowledge of any formation in the region which is 

 older than the limestone series. 



It has been suggested that the metamorphism of the limestone is a 

 result of the intrusion of the gabbro masses which are so extensive in 

 the Adirondack region; but the great extent of the metamorphosed 

 series, the completeness of crystallization in all parts, the narrow and 

 sharply defined contact-zones and the comparatively small quantity of 

 known intrusive rocks combine to render the explanation rather im- 

 probable as applied to this particular area. On the other hand, the evi- 

 dences of great meclianical disturbances justify the conclusion that the 

 metamorphism is largely dynamic. 



