144 NEW YORK STATE MUSEUM 



THE DARK COLORED, BASIC ROCKS 



BASIC ROCKS IN THE ADIRONDACKS 



Traps or diabase dikes occur in great numbers in the main 

 Adirondack region, though they are very unequally distributed. 

 They occur with greatest frequency in the eastern and northern 

 parts, embraced in Essex, southwestern Clinton and southern Frank- 

 lin counties. As they were intruded during the Precambric they 

 are not found outside the area underlain by the crystalline forma- 

 tions — the gneisses, schists, crystalline limestones and plutonic 

 igneous masses ; but they may be looked for in any of the rocks 

 just named. 



The Precambric area of Essex and Clinton counties includes 

 numerous examples of the dikes, so many that their separate occur- 

 rence has hardly seemed worthy of note in the geological reports 

 dealing with this section. They are particularly in evidence in the 

 vicinity of the iron mines at Hammondville, Mineville, Ausable 

 Forks, Lyon Mountain and in the Saranac valley ; but are probably 

 no more frequent there than elsewhere in the same region; they 

 are simply better exposed. The writer has noted more than a 

 hundred such dikes in these districts. They all present very similar 

 features of physical development, consisting typically of feldspar, 

 pyroxene and magnetite with the peculiar diabase texture which 

 arises from the inclusion of the pyroxene within the meshes formed 

 by the interlacing feldspar laths. As a rule they are fairly fresh at 

 the surface and give a metallic ring when struck with the hammer. 

 They have the tabular form characteristic of fissure intrusions and 

 are seldom more than a few feet thick though persistent on the 

 line of strike. Their prevailing direction is from north to north- 

 east in conformity with the main structural trend of the inclosing 

 rocks. The trap is well suited for road material on account of 

 its toughness and wearing qualities, but the occurrences so far dis- 

 covered are scarcely of sufficient size to justify quarry work. Dikes 

 over 15 feet thick are very rare and of those seen by the writer a 

 thickness of 30 or 40 feet represents about the maximum. They 

 have a steep dip, usually nearly vertical, so that their quarrying 

 would be difficult and relatively expensive. 



A more available material for local road building in the Adiron- 

 dacks is found in the areas of gabbro and basic syenite. The latter, 

 normally a feldspathic rock, develops in places into a very dark 

 material with abundant iron-magnesia minerals and magnetite, 

 which closely resembles and even grades into the gabbros. The 



