ANCIENT VOLCANIC ROCKS. 55 



becomes replaced by the " microfelsitic " substance, the 

 original perlitic structure being sometimes still traceable, 

 like the veins in the mesh-structure of serpentine derived 

 from olivine. More interesting is the tracing of a passage 

 from this type into the " Dobritz porphyry," which had 

 formerly been regarded as a quite distinct rock. In the 

 microcrystalline ground of the latter rock occasional relics 

 of the perlitic structure are still preserved, as well as the 

 microlites of iron-ore, now converted to limonite, and in 

 places spherulites. The evidence of conversion of a glassy 

 to a cryptocrystalline and finally to an evidently micro- 

 crystalline rock seems to be complete at every point. 



An interesting question raised is that of the relation of 

 devitrification to dehydration. The author considers that 

 in the first stage of devitrification of the pitchstone there 

 has been, not a loss, but a certain gain of water. In the 

 passage to the microcrystalline state, however, the rock has 

 lost most of its water, and the consequent contraction has 

 eiven rise to crevices and hollows. 



The so-called porphyrites of the district vary from what 

 is described as a normal mica-porphyrite to one compara- 

 tively rich in quartz. They are almost all greatly decom- 

 posed ; but one, termed porphyrite-pitchstone, is relatively 

 fresh, with an unaltered glassy base. It contains enstatite, 

 as well as plagioclase, quartz, hornblende, and biotite, and 

 if it occurred in Hungary would doubtless be named 

 enstatite-dacite. The author gives reasons for believing 

 that this rock represents the original type of the whole 

 group, the microcrystalline ground-mass of the majority 

 beine a result of devitrification. His researches therefore 

 go to establish that not only may quartz-porphyries (in the 

 descriptive sense) be derived from glassy rhyolites, but also 

 quartz-porphyrites from glassy dacites or quartz-andesites. 



The most complete account yet given of a limited group 

 of ancient volcanic rocks is perhaps Mugge's(6) description 

 of the so-called " Lenneporphyre". These rocks, occurring 

 in the Middle Devonian of the Lenne district in Westphalia, 

 have been the subject of much discussion. Mugge shows 

 conclusively that they are contemporaneous igneous rocks, 



