1893.] T. H. Holland — Slab of Chinese Agglomerate Lava. 16 5 



to be a siliceous lava, which, though of course formed by natural means 

 is, indeed, comparable to a concrete in ways other than appearances. It 

 is a rhyolitic lava of a kind occurring in different parts of China, which, 

 previous to consolidation, has included fragments of other rocks and 

 now presents the patchy appearances of the agglomerate lavas and pi- 

 peruos described by Fritsch and Reiss as varieties of eutaxite.* 



Thin slices examined under the microscope leave no doubt as to the 

 nature of the rock : — Corroded quartz-crystals embedded in a cryptocrys- 

 talline and microlitic magma are scattered irregularly through the slide. 

 Occasionlly these preserve in part their original bi-pyramidal outlines, 

 but the magma has corroded the majority of the crystals into irregular 

 shapes. A curious feature worthy of record is the way in which many 

 of the quartz-crystals are traversed by a series of cracks without discov- 

 erable regularity. These cracks recall the tessellated appearance of the 

 polysynthetic porphyritic crystals described by Gen. McMahon in the 

 eurite of Tusham Hill, 85 miles north-west of the town of Delhi. f But 

 as a rule, in the present instance, the small fragments, although separated 

 from one another by a series of cracks, all have the same optical 

 orientation, whilst in the Tusham specimens the grains are, according to 

 Gen. McMahon, oriented in different directions. I have found, however, 

 one case of a quartz -crystal in which, after the formation of the cracks, 

 many of the fragments have been slightly displaced, so that whilst the 

 position of extinction is the same for the individuals in some of the pairs, 

 others show slight differences, and still others have been moved through 

 several degrees. Gen. McMahon explained the structure of the quartz- 

 crystals in the Tusham rock as the result of rapid cooling after eruption, 

 and I think the present case, in which many of the crystals are simply 

 cracked more often without displacement of the fragments, are certainly 

 more easily explained in this manner than by the other suggestions 

 which, in his paper, Gen. McMahon has considered and rejected. Relief 

 of pressure would also contribute to the same effect. A similar struc- 

 ture can be produced in clear quartz- crystals by rapidly cooling them 

 from a red heat, the crystals becoming white and losing their transparency 

 from a similar cause. 



Next to the quartz-crystals in abundance amongst the porphyritic 

 constituents are the felspars, some of which are of a plagioclase variety, 

 and all greatly kaolinized. Black and brown patches of ferruginous 

 material occur as relics of the ferro-magnesian constituents of the ori- 

 ginal rock. Secondary minerals like chalcedony occur in small quanti- 

 ties infilling cavities. 



* Geologische Beschreibung der Insel Tenerife, 1868, p. 420. 

 t Ik Mag., Vol. VIII. (1683), p. 10. 



