BUILDING AND ORNAMENTAL STONES. 433 



According to Zirkel, the average chemical composition of diabase is 

 as follows : 



Per oeut. 



Silica 49.54 



Alumiua 14.05 



Iron protoxide 14.27 



Lime 8.20 



Magnesia 5. 28 



Potash 1.16 



Soda 3.88 



Water 2.29 



Average specific gravity, 2.8, equal to a weight of 175 pounds per 

 cubic foot. 



In classification two principal varieties of diabase are recognized, the 

 distinction being founded upon the presence or absence of the mineral 

 olivine. We thus have (1) olivine diabase, or diabase with olivine, and 

 (2) diabase proper, or diabase without olivine. 



Owing to its lack of definite rift, compact texture, and hardness, dia- 

 base can, as a rule, be worked only with difficulty and usually at a cost 

 considerably greater than that of granite. It is therefore not exten- 

 sively quarried, though of late years it has come into more general use 

 for paving purposes, and still more recently for building and monu- 

 mental work. The green antique porphyry or Marmor Lacedwmonium 

 viride, formerly much used for pavements and general inlaid decorative 

 work in Greece and Rome, is, according to Delesse,* a diabase consisting 

 of large greenish crystals of labradorite embedded in a fine compact 

 ground mass of the same feldspar, together with augite and titaniferous 

 iron. The quarries from which the stone was taken are stated by Hullf 

 to be situated between Sparta and Marathon, in Greece. A stone of a 

 similar character and closely resembling it in color and structure is 

 abundant among the drift bowlders of eastern Massachusetts, but its 

 exact derivation is unknown. 



In the eastern United States the dikes of diabase are frequently as- 

 sociated with deposits of red or brown Triassic sandstone, which are also 

 extensively quarried, as will be noticed further on. Concerning these 

 dikes Professor Dana writes: % 



"It is remarkable that these fractures (through which the diabase 

 was forced to the surface) should haA^e taken place in great numbers 

 just where the Triassic beds exist, and only sparingly east or west of 

 them j and also that the igneous rock should be essentially the same 

 throughout the thousands of miles from Nova Scotia to North Carolina. 

 The igneous and aqueous rocks (sandstone) are so associated that they 

 necessarily come into the same history. Mount Tom and Mount IIol 

 yoke, of Massachusetts, are examples of these trap ridges; also East 

 Rock and West Rock, near New Haven, and the Hanging Hills, near 



*Annals de Mines, p. 256. 

 tOp. cit, p. 73. 



t Manual of Geology, third edition, p. 417. 

 H. Mis. 170, pt. 2 28 



