56 SCIENCE PROGRESS. 



in part lavas, in part fragmental accumulations. The lavas 

 are distinctly acid rocks with a general preponderance of 

 soda over potash, and may thus be described as rhyolites 

 or soda-rhyolites. Following the German terminology, the 

 author styles them Keratophyre. They are divided into 

 quartz-ceratophyres and felsite-ceratophyres, according to 

 the presence or absence of porphyritic quartz, although the 

 latter rocks are scarcely less acid than the former. One 

 type has abundant porphyritic crystals of corroded quartz 

 and alkali-felspars with flakes of pale mica, which is not 

 muscovite but a bleached biotite. There are sometimes 

 large nodular bodies (giant spherulites) with a central 

 hollow, and there may be relics of a spherulitic structure on a 

 smaller scale, but the ground-mass is believed to have been 

 largely glassy. It has now completely lost its original 

 characters, and in some cases consists mainly of quartz 

 granules. There has evidently been not mere devitrifica- 

 tion, but an introduction of silica, which is proved by the 

 high silica-percentage of the rock, reaching as much as 82^. 

 There are little veins of opal in the ground, as well as 

 quartz. From this massive rock every gradation is seen 

 to a schistose and very sericitic kind of the so-called " por- 

 phyroid," which here, as elsewhere, is proved to result from 

 crushing. 



Another type is much poorer in porphyritic crystals. 

 The around-mass, though containing; veins and nests of 

 secondary quartz, is richer in orthoclase than the preceding, 

 and usually shows little altered spherulites. 



The felsite-ceratophyres lack the quartz and mica of the 

 former types. Soda preponderates greatly over potash in 

 the analyses, and the silica-percentage is about 79. All 

 these rocks show a strongly marked flow-structure, which 

 passes uninterruptedly through certain large skeleton- 

 spherulites. The latter (closely paralleled in some of the 

 Welsh Ordovician rhyolites) sometimes make up almost 

 the whole bulk of the rock. These rocks too are subject 

 to crushing, and, as in the former types, the schistose 

 varieties have not the high silica-percentage of the un- 

 crushed. 



