348 NEW YORK STATE MUSEUM 



cambric) in the upper Great lakes region, and it is thought that 

 they are of approximately the same age. 



In only one case has it been possible to make a relative age 

 determination between the two sets of dikes. Near the summit 

 of Rand hill a 15 inch dike of the syenite porphyry, bearing 

 n.65°e., is cut by a diabase dike of the same width bearing e= 

 and w. In this case the diabase is indisputably the younger. 

 While this does not demonstrate that all the diabase is younger 

 than all the syenite porphyry, it at least points strongly to such 

 a conclusion. 



Syenite porphyries. The rocks from the various dikes differ 

 considerably. Nearly all of them show porphyritic feldspars, 

 though with much variation in size and abundance. These are 

 usually red, but become greenish in one dike, and in another are 

 of a red violet hue. Except in oue dike this is the only porphy- 

 ritic mineral, biotite also appearing in this case. The dikes are 

 often of pronounced red color, but some are much darker, gray 

 to black, with often a greenish tinge when slightly altered. The 

 narrower dikes are dense, hard rocks with conchoidal fracture 

 and aphandtic appearance and general red color. The larger ones 

 are equally hard and firm, but coarser grained and less apt to 

 be red. 



These rocks are essentially composed of microperthitic feldspar 

 and biotite, with accessory magnetite (or specular hematite), 

 hornblende, quartz, albite, orthocla.se, microcline, apatite and 

 titanite, and with secondary chlorite, calcite, muscovite, epidote 

 and hematite. Microperthite and chlorite, the latter from biotite 

 alteration, ai'e the only minerals present in all the dikes. Quartz 

 is present in most of them, in quantity varying with the com- 

 position. The ground-mass has in general a well marked flow 

 (trachytic) structure.^ 



They show a surprising range in composition considering their 

 rather constant mineralogy, varying from the acidity of granites 

 to, in the case of one dike, a basicit}^ approaching that of basalt. 



Diahases. The numerous dikes of this rock in the Adiron- 

 dacks exhibit many variations in composition and texture, and 

 on the northern slopes of the north foothills of the region, where 

 glacial erosion was powerful and all weathered rock was swept 



^For detailed description, see Geol. Soc. Am. Bui. 9 :239-56. 



