GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



349 



away, they are often found in beautifully fresh condition. They 

 have not, as yet, received the thorough description which they 

 merit, Kemp's account of them being the most exhaustive which 

 has vet appeared. 1 



The usual diabases consist essentially of a plagioclase feldspar, 

 mostly either andesin or labradorite, augite and magnetite. To 

 these olivin must be added in a very large portion of the dikes, 

 the number of olivin diabases equaling or exceeding that of those 

 without this mineral, so far as the writer's observation goes. 



The smaller dikes are, almost without exception, porphyrinic, 

 and the same is true of at least the borders of the larger ones, 

 though these frequently become sufficiently coarse grained in 

 their central portions to cause this character to lose its distinct- 

 ness. As a general proposition, the dikes may be said to be 

 characterized by two generations of one or more of the minerals 

 present, sometimes the feldspar alone, sometimes the augite 

 alone, sometimes both, occurring in this way. The olivin, when 

 present, seems always to belong to the first generation. 



Three of the Franklin county dikes are notable in containing 

 an orthorhombic pyroxene, bronzite, in considerable quantity. It 

 is porphyritic in all, and with its coming in, olivin retreats. In 

 two of them, it gives rise to beautiful parallel growths with 

 augite of a certain sort, nearly all the bronzites being bordered 

 by a narrow zone of this mineral, after the usual law of such 

 growths. The augite plainly did not begin to form till the period 

 of bronzite formation had passed, and the crystals furnished 

 nuclei favoring augite growth. 



In some 25;£ of the dikes biotite is present, occurring in 

 frequent small scales in the ground-mass, with a notable tendency 

 to border the magnetite crystals. In such situation it has been 

 sometimes regarded as primary and sometimes as a result of 

 magnetite feldspar corrosion. Kemp looks on it as the former, 

 in the Essex county dikes. The writer has been unable to satisfy 

 himself as to which view is the proper one, in the case of his own 

 dikes, though disposed to the latter view. 



That these rocks show a notable range in composition is 

 indicated by the considerable variation in the relative amounts 

 of feldspar and augite, the former being very materially in excess 



X U. S. Geol. Sur. Bui. 107, p.24-27. 



