50 SCIENCE PROGRESS. 



showed that hypersthene in small crystals of very different 

 appearance is a constituent of widespread occurrence in 

 Tertiary andesites, and it has since been recognised as 

 equally abundant in andesites of Palaeozoic age. 



The minerals which have been considered peculiar to 

 the Tertiary volcanic rocks are leucite, nosean, hauyne, 

 melilite, and tridymite, and some of these are still scarcely, 

 if at all, known in the older lavas. Considering, however, 

 the rarity of melilite-basalts and haiiyne-bearing rocks 

 among the Tertiary and Recent lavas, it is not a matter for 

 surprise that they should remain unknown among lavas 

 which have received less attention. 



What is perhaps more significant is the fact that all 

 these minerals are peculiarly liable to decomposition, and 

 of some of them, at least, all trace would easily be obliter- 

 ated by secondary changes in the rock. The case of 

 leucite is especially interesting, for here, although the 

 mineral is commonly destroyed in the older rocks, its un- 

 mistakable crystal-form is in some instances clearly pre- 

 served by its decomposition products. Of this nature seem 

 to be the "pseudo-crystals" composed of orthoclase and 

 nepheline described by Hussak and Derby in the phonolite 

 and foyaite of Tingua Mountain in Brazil. Opinion was 

 somewhat divided as to the nature of these pseudo-crystals, 

 but doubts may be considered to be set at rest by the study 

 of similar pseudomorphs in other districts, and especially 

 by J. F. Williams' description of the leucite-syenite of 

 Magnet Cove in Arkansas. If the large leucite-crystals 

 in such rocks have become obscured, it is easy to under- 

 stand how the small crystals of volcanic rocks may have 

 been altogether obliterated. 



Palaeozoic leucite-lavas, however, are not wholly un- 

 known, and one has been described by Von Chrustchoff (i) 

 from Siberia. It occurs on the right bank of the Tunguska, 

 forming a flow distinctly overlain by limestones containing 

 Favosites, Haly sites, Cyathophyllum, and other characteristic 

 Lower Palaeozoic fossils. Little white crystals of leucite, 

 up to i mm. diameter, are visible in a compact ground- 

 mass, and the microscope shows augite, anorthoclase, sani- 



