432 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



PETROGRAPHICAL DESCRIPTION. 



DIABASE-PITCHSTONE. 



The diabase-pitchstoue in its purest form is a dark liver-brown pitch- 

 stone, dull-green or mottled brown and green by reflected light, and red- 

 brown by transmitted light. It is often an apple-green glass with the same 

 dichroism. It has resinous pitchstone luster, and so differs from most taehy- 

 lytes. The microscope shows a very minute, regular network of cracks, 

 often developing into a perlitic structure around crystals and spherulites, 

 which explains this luster. The deep-brown glass streaked with very deep 

 brown is wholly amorphous and hardly to be distinguished from the Kilauea 

 glass in common light, and, like it, it is not affected by acid. The pheno- 

 crysts are of similar size and distriljution, but with polarized light the feld- 

 spar rods are always, and the large colorless pyroxenes sometimes, changed 

 to granular calcite, easily removed by acid; the olivines, to fibrous serpentine. 



The fresh glass is full of small grains (cumulites), white by reflected 

 light, red-brown by transmitted light, which are made of aggregates of 

 minute grains (globulites). Even where the glass seems compact it often 

 separates into small sheets and portions, showing minute curdled surfaces, 

 and under the microscope the same wrinkled surfaces can be seen where 

 small cavities have collapsed or where the fragments have flowed or have 

 been drawn out in threads. 



The glass has been shattered into angular fragments by sudden explo- 

 sion while still able to flow under slow pressure. Each of the fragments 

 is then bordered by a layer of even thickness of paler-brown and equally 

 nonpolarizing glass — an effect of the heated waters on the iron content. 



The larger fibrous spherulites in the glass are usually perfect circles 

 or ovals, but they are sometimes distorted by flow or pressiu-e. They are 

 often bordered by several concentric bands of lighter and darker brownish- 

 green glass, each band having a concentric radiate structure. The central 

 part is colorless and beautifully radiate-fibrous, showing perfect black cross. 

 The fibers are optically positive and polai'ize like a plagioclase. They are 

 not affected by boiling acid or alkali. Sometimes the centers are filled hv 

 a greenish granular mass, which scarcely polarizes, showing only scattered 

 light points. The spherulites are often broken and found in parts in the 

 breccia, and the layers separated and crushed, so that the glass seems full of 



