274 SMYTH — ROCKS OF NORTHWESTERN ADIRONDACK REGION. 



by narrow bands of fragmental grains, while the large individuals have 

 very strong undulatory extinction. From such cases there is a gradation 

 up to a complete crushing of the rock to a mass of minute grains, with 

 only here and there a small residual core. 



Chemical Analyses. — The analyses given below show that the microper- 

 thitic variety is rather too acid to be classed as a member of the gabbro 

 family, as might be expected from its mineralogic aspect in thin sections, 

 but there is no doubt of its geologic continuity with the more basic 

 phase. They are different portions of a single intrusive mass and owe 

 their differences to magmatic variations. It is by no means easy to de- 

 termine in the field which is the prevailing type, but the facts at hand 

 point strongly to the preponderance of the basic rock ; hence the use of 

 the name gabbro for the intrusion. In fact, the employment of this term 

 would be justifiable even were the acid phase more abundant in this 

 particular mass, inasmuch as the presence of a large quantity of the 

 basic phase points so strongly to the conclusion that the entire mass 

 belongs to the great gabbro intrusions of the region. 



I. II. 



Si02 57.00 65.65 



AlA 16.01 16.84 



FeO 10.30 4.01 



MgO 1.62 0.13 



CaO 6.20 2.47 



K2O 3.53 5.04 



NaaO.... . 4.35 5.27 



H2O 15 .30 



Totals 99.16 99.71 



I. Average sample of the ordinary basic variety ; from Natural Bridge. 



II. Acid variety, in section, largely composed of microperthite ; from near Har- 

 risville. 



RE LA TION OF GABBRO TO LIMESTONE. 



Character of the Transition. — As previously stated, the northern bound- 

 ary of the gabbro belt is clearly defined, being the line of its contact with 

 the crystalline limestone series. It is not meant to imply that the pre- 

 cise location of this boundary has been determined throughout its entire 

 length, for such is not the case, but merely that there is nothing like a 

 transition between the two formations, the passage from one to the other 

 being abrupt. As some portions of the gabbro might be taken for meta- 

 morphosed sediments, it is important to establish the true nature of this 

 contact in order to justify the conclusion that the rock is igneous. 



Character of the Contact. — Many absolute contacts have been examined, 

 while in even more cases the two formations have been observed very 



