40 



NEW YORK STATE MUSEUM 



The whole range in mineralogical composition is brought out in 

 the following table : 



Table 9 

 Mineralogical composition of the diabase 





Slide 

 no. 



Andesine 



to labra- 



dorite 



Augite 



Biotite 



Ilmenite 





Glassy 



ground 



mass 



Quartz 



Apa- 

 tite 



I 



8 

 48 



7 



10 

 9 



47 

 55 



55 



5 

 55 



25 



22.5 

 57 



4 

 6 



5 



many 



specks 



5 



little 

 little 



85 

 25 



Httle 



2 



little 



I 



little 

 Httle 



little 



2 



Mostly chlorite 

 40 



3 



4 



5 



Mostly 



5 

 14 



chlorite. 



The remarkable similarity in composition is a striking feature. 

 Nos. I, 2, and 3 are typical holocrystalline diabases from widely 

 separated dikes. Nos. 4 and 5 represent finer grained or border 

 phases and have more or less glassy ground mass. No. 5 presents 

 a striking appearance under the microscope because the feldspar 

 crystals which are incipient and almost indeterminate tend toward 

 sheaflike bundles (see plate 6, lower figure). 



Number i of table 9, which may be regarded as typical of all the 

 diabases, is from the large dike (above described) at the base of 

 Heath mountain. The fine to medium grained rock shows an excel- 

 lent diabasic texture visible even to the naked eye. Judging by the 

 extinction angles, the broad laths of somewhat decomposed plagio- 

 clase range from andesine to labradorite in composition. Pale red- 

 dish brown augite, in stout prisms, shows a very faint pleochroism. 

 It exhibits good cleavage and sometimes good crystal boundaries. 

 The biotite is much changed to chlorite and stained with black iron 

 oxid. The ilmenite often shows transition to leucoxene. Apatite 

 occurs in tiny needles, and pyrite and quartz in small irregular 

 grains, the latter being probably of secondary origin. 



Chemical composition, norm, and mode 

 The diabase from the dike at the western base of Heath moun- 

 tain has been analyzed for the writer by Prof. E. W. Morley. 



