A UTHORS ' ABSTRA CTS I 1 9 



and with faces even but not polished or showing any indication of 

 rubbing, such as are common in the region as a whole, are found reg- 

 ular oblique joints, cutting the roof and floor as well as the coal, and 

 having their faces much slickened. These joints are evidently shear- 

 ing planes, and show signs of incipient movement, or in some cases, 

 as in one of the figures given, they become fault planes allowing the 

 lower of two coal beds twenty feet apart to slide an unknown distance 

 up over the upper bed. The main system of joints dip to the north, 

 a second set sometimes developed dip to the south. The pressure 

 acted with the general strike of the rocks. The area described covers 

 a few square miles southwest of Asherville, Clay county, Ind. The 

 structure has been disclosed by the operations of mining. 



Syenite-porphyry Dikes in the Adirondacks. By H. P. Cushing. 



Interesting dikes of pre-Potsdam age have been recently found in 

 the Adirondacks which constitute the complementary rocks to the 

 diabases of the region. Basal conglomerates of the Potsdam contain 

 pebbles derived from them. On the other hand, together with the 

 diabases, they comprise the only unmetamorphosed pre-Potsdam rocks 

 known in the district. 



These dike rocks are made up of feldspar, quartz, and biotite, with 

 or without hornblende, and with accessory iron ores, apatite, augite, 

 titanite. Both orthoclase and albite feldspar are present, commonly 

 intergrown as microperthite. The rock is porphyritic, both feldspar 

 and biotite occurring in two generations, the latter only sparingly. 

 The feldspar largely predominates over all other minerals, constitut- 

 ing from 60 to 80 per cent, of the rock. The ground mass structure 

 is trachytic or orthophyric. 



Chemically the rocks are characterized by high alkali and low lime- 

 magnesia percentage, and rather low ratio of alumina to silica. They 

 belong to the alkali-syenite group of Rosenbusch. Their bunched 

 distribution and mineralogic similarity indicate that they were pro- 

 ducts of a common magma, and if that be so they afford an interesting 

 case of magmatic differentiation as they range from 69 to 52 per cent, 

 of silica. With decreasing silica the lime, magnesia and alkali per- 

 centages rise, the latter retain their preponderance. 



The greater number of the dike rocks would be classed as syenite- 

 porphyries, nordmarkite, and pulaskite types. The more acidic repre- 



