494 J. A. PHILLIPS ON THE SO-CALLED " GEEENSTONES " 



their substance is replaced by hornblende exhibiting all the dis- 

 tinctive characteristics of that mineral ; the rock thus passes into a 

 uralite-dolerite or " uralite-diabase." 



When the augite in rocks undergoing this species of alteration 

 does not occur in well-formed crystals, the sharp line of division 

 which exists between the altered and unaltered portions might lead 

 to the conclusion that a mixture of hornblende and augite was ori- 

 ginally present. The examination, however, of sections containing 

 well-crystallized augite shows that, in addition to this mineral in a 

 partially altered state, perfectly formed crystals of uralite are also 

 frequently present. It therefore follows that, although both augite 

 and hornblende may have sometimes been original constituents of 

 certain rocks, the alteration of augite into uralite has in other cases 

 actually taken place. 



A microscopical examination of above two hundred carefully 

 prepared sections from rocks of this class proves that a very large 

 majority of them are altered dolerites. It would, however, have 

 been impossible to classify many of the more altered specimens, had 

 not an extensive series, exhibiting various gradations of change, been 

 available for comparison. 



When rocks of this class do not contain augite, and are to a very 

 large extent composed of long bacillar hornblendic crystals made 

 up of parallel belonites, of which the ends are frequently curved 

 outwards, it is possible that this mineral may sometimes have been 

 an original constituent. 



The rocks both at Scaddick Tor and at Trewint belong to this 

 class ; but on referring to the analysis of the latter, it will be ob- 

 served that in chemical composition it closely agrees with the 

 altered dolerites of Penzance. 



At Pope's Mill, near Liskeard, and at Wearde, in the neighbour- 

 hood of Saltash, the crystalline traps contain mica, in addition to 

 their more ordinary constituents. 



Slaty or schistose greenstones are of less frequent occurrence in 

 the eastern and central districts than they are in the more western 

 portions of the county. On St.-Cleer Downs, however, the " horn- 

 blende-slates " graduate imperceptibly from crystalline dolerite on 

 the one hand, into clay-slates on the other; and instances are by no 

 means wanting where a rock is massive and crystalline near the 

 centre of its outcrop, while externally it is schistose and without 

 visible crystals. These slaty greenstones, or hornblende- slates, as 

 they have been sometimes called, are here exceedingly hard, and 

 have often a structure as compact as that of quartzite ; they are 

 somewhat foliated, and break with greater facility along certain 

 planes than in other directions. A microscopical examination of this 

 rock shows it to consist mainly of a transparent colourless material 

 containing grains of quartz, through which are thickly disseminated 

 flakes of a hornblendic mineral, not averaging more than -05 millim. 

 in diameter ; it also contains unaltered titanic iron, and a nebulous 

 green substance which high powers resolve into imperfect crys- 

 talline forms. On comparing the analysis of this rock with that of 



