562 Report of the State Geologist. 



• 



Such indecisive results are presented very apologetically. The scarcity 

 of fossils and the lark of time for work of the detailed character necessary are 

 accountable theref< >r. 



Series V. The eight dikes found in Beekmantown, are all on Rand 

 hill, cutting the Pre-Cambrian rocks. The proportion of bostonites is greater 

 than usual, six of the dikes being of that type. These show well the very 

 considerable variation which such rocks present. Dikes Nos. 28 and 29 

 are of dense, hard red rock of aphanitic, stony look, almost entirely devoid of 

 phenocrysts, and with very small content of ferro-magnesian silicates. Nos. 

 27 and 107 have a peculiar schistose appearance in the hand specimen, as if 

 tlie\ had been subjected to shear, but the appearance is entirely lost in the 

 thin section, which appears quite normal. They contain a larger proportion 

 of dark silicates than the first two. No. 27 has also abundant orthoclase 

 phenocrysts of unusual purplish color, which is due to the rilling of the 

 cleavage cracks by hematite. 



Nos. 31 and 103- are nearly black rocks, likely to be taken for diabases 

 in the field unless carefully inspected, but in the slide are <piite like the 

 others except for the presence of a much greater amount of hornblende and 

 biotite. These occur "mainly as inclusions in the orthoclase, and the trachytic 

 structure is not well denned. 



Series VI. Rand hill is so heavily banked with glacial deposits on the 

 east, south and west, that all rock is obscured by them for a distance of at 

 least two miles in these directions, and commonly for more. The surface is 

 everywhere strewn with loose irregular blocks of Potsdam sandstone. These 

 accumulations may be morainicin origin, as suggested by Mi-. Baldwin,* but 

 no opportunity w as afforded for closely investigating them. 



Altona. 



Series I. Gneiss of the ordinary microperthitic variety is exposed on 

 the eastern side of Rand hill. Its contact with the anorthosite is show n 

 along the turnpike (old military road) and exhibits a transition zone, a few 

 feet in width, of a rock of intermediate mineralogic constitution. This 

 transition rock has cataclastic structure and shows a dikedike dark band 

 w hich proves to be a sheared strip, consisting of a finely granular aggregate 

 of quartz, microperthitic orthoclase, augite, garnet, and a little plagioclase. 

 The garnet is certainly secondary, and much of the rock has undergone 

 recrystallization. 



♦American Geologist, Vol. XIII., p. 1H1. 



