180 DESCRIPTIVE GEOLOGY. 



from 3 to 4 miles in length, perfectly straight, and almost continuous, varying 

 in height from 30 to 60 feet. It is 6 feet in width throughout, either side 

 being perpendicular, while its summit presents shapes of square towers and 

 bastions, whence its name, ''The Rampart". It is made up of a columnar 

 basalt, whose columns are arranged horizontally and at right angles to the 

 length of the dike. This basalt is as remarkable as the dike itself, being 

 a light-gray crystalline, rather porous, rock, full of fine crystals of black 

 mica and having something the appearance of a trachyte. Under the micro- 

 scope, it shows neither triclinic feldspar, hornblende, nor olivine, but a great 

 quantity of yellowish-green augite crystals, and a colorless, un striated 

 ingredient which the chemical tests show must be nepheline, together with 

 considerable apatite. Part of the colorless material is left untouched by 

 acids after long digestion, and must therefore be sanidin-feldspar. 



