476 PROCEEDINGS OP THE NATIONAL MU8EVM. vol.37. 



Ste})()vak Bay (20)." The majority of these descriptions are very 

 brief; some inchide petrograpliic details, but very few contain sucli 

 important essentials as chemical or mineralogical composition. 

 Most of the occurrences, too, represent rocks more or less changed by 

 alteration. 



In view of the above considerations, a detailed cjuantitative descri])- 

 tion of an olivine-diabase of exceptional freshness and unusual 

 richness in olivine is herein given. The material is represented in 

 the U. S. National Museum collections by deposit No. 77422. 



Field occurrence. — Olivine-diabase occurs in Davidson County, 

 North Carolina, in the form of dikes which cut a series of ancient 

 volcanic rocks. This series includes slate, acid tuffs and breccias, 

 rh^^olite, dacite, andesitic tufl's and breccias, andesite, and dikes of 

 gabbro and diabase, and is a part of a broad band of volcano-sedi- 

 mentary rocks, called the ''Carolina slate belt," which crosses the 

 State in a northeast-southwest direction, forming an important part 

 of the Piedmont Plateau. The dikes are iniiformly, though not 

 abundantly, distributed, and show upon the surface as narrow lines 

 of rounded, yellowish bowlders, locally called "nigger-heads." They 

 vary in size from a few feet in width and a few yards in length to a 

 hundred feet in width and over a mile in length. The majority 

 conform to the former rather than to the latter dimensions. In 

 trend they also vary, but the more common directions are included 

 between N. 30° E. and N. 30° W. 



The olivine-diabase is doubtless of Triassic age, for dike rocks of 

 similar character have a widespread occurrence throughout the Pied- 

 mont Plateau, and in many places may be traced into areas of Triassic 

 sandstone. 



Alegascopic descriftion. — The olivine-diabase is a massive, fine- 

 grained, dark-blue rock, with a faint jmrplish tinge and a more or 

 less wax}^ luster. To the unaided eye it appears a closely knit 

 aggregate of dark-colored minerals, showing numerous small crystal 

 faces. With the hand lens it is possible to recognize occasional 

 striated feldspars, and to distinguish from these the darker-colored 

 ferromagnesian constituents. The augite and olivine, however, can 

 not be differentiated; this is notable in view of the fact that the latter 

 mineral comprises nearl}^ one-fifth of the rock. 



Microscopic description. — The microscope reveals the following 

 minerals, named in the probable order of their formation: Iron ore, 

 olivine, plagioclase, and augite. (See pi. 37.) 



"Many of the rocks along the Atlantic coast formerly described as "trap," under 

 present usage woiild be termed olivine-diabase; but an attempt to differentiate these 

 would involve too long a discussion for the present purpose. 



