PETROGRAPHY OF THE MERIDEN " ASH BED. 73 



microlites, which are brought out with bluish white light by crossed 

 nicols. They are red brown by transmitted light from the ferrite grains 

 of the former brown glass, crowded between the rods, and silvery white 

 by reflected light. 



Exquisite tufts and sheafs of feathery, beaded microlites overlap each 

 other to form an opaque ground, except in thin slides. They reproduce 

 the structures figured by E. S. Dana from Kilauea.* Beautiful speci- 

 mens for slide \ can be obtained from the great mass of liver brown 

 aphanitic trap on the bluff just above the blasting. 



Diabase pitchstone. — Mr George W. Hawes was apparently the first to 

 mention the occurrence of glass and the last described variety of diabase.f 



In its purest form, as found in a great block at the top of the bluff, it 

 is a dark liver brown pitchstone, dull green, or mottled brown and green, 

 by reflected and red brown by transmitted light. In the cliff wall below 

 it is often an apple green glass with the same dichroism. It has resinous 

 pitchstone luster, and so differs from most tachylites. The microscope 

 shows a very minute, regular network of cracks, often developing into a 

 perlitic structure around c^stals 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 phenocrysts 

 are of similar size and distribution, but with polarized light the feldspar 

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

 granular calcite, easily removed by acid ; the olivines to fibrous serpen- 

 tine (see plate 8, figures 4 and 5). 



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

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

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

 rates 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 surface of fragments have 

 flowed or have been drawn out in threads. In one case a large fresh 

 pyroxene resting alone in the fresh brown glass has been bent 35 degrees 

 without fracture during the torsion of the glass fragment (see plate 8, 

 figure 5). 



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

 sions, while still able to flow under slow pressure. Each of the frag- 

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

 equally non-polarizing glass, an effect of the heated waters on the iron 

 content (see plate 8, figures 4 and 5). 



*Am. Jour. Sci., vol. xlvii, p. 441. 

 fProc. U. S. Nat. Museum, vol. iv, p. 129. 



