418 S. ALLPOKT ON THE METAMOKPHIC KOCKS 



II. Metamorphic Rocks of Igneous Origin, 

 Altekeb Dolekites. 



The rocks remaining to be described are the " Greenstones" of th© 

 Geological-Survey map ; a few of them would doubtless be regarded 

 as diabase or melaphyr by some authors, and since the publication 

 of the geological map they have even been described as nothing but 

 altered slates. They appear to have been correctly mapped, and for 

 the most part rightly named, by the officers of the survey, according 

 to their use of the term greenstone. These rocks vary in colour 

 from a dark bluish green to dark brownish green, and afford several 

 varieties of texture. Some specimens from Tolcarn, Penlee Point, and 

 elsewhere are rather coarsely crystalline ; they are not fissile in any 

 direction, and yield with difficulty ^to repeated blows of the hammer j 

 while other portions of the same masses are either fine-grained or 

 compact, and possess an imperfect slaty cleavage. The coarsely 

 crystalline rocks are, as I shall presently show, altered dolerites; 

 while some, if not all of the more compact slaty varieties were 

 originally fine-grained basaltic portions of the same rocks, but have 

 been in most cases so highly metamorphosed as to be almost inca- 

 pable of recognition. These dark-coloured rocks are locally known 

 as blue or grey elvans, and differ greatly in appearance from the 

 "killas" in which they occur; their stratigraphical relations are, 

 however, not quite clear. All the rocks of the district have been 

 greatly disturbed ; and it would require more time than I had at my 

 disposal to determine whether these masses of trap are contempo- 

 raneous and interbedded, or intrusive sheets. Probably both occur,, 

 as frequently happens in volcanic districts, and may thus, in either 

 case, belong to the same period as a whole; but, however this may 

 be, they are certainly older than the granite. 



A good example of the coarsely crystalline variety occurs in an 

 old quarry at Tolcarn, between Penzance and Newlyn, where it may 

 be seen to overlie the slate ; and on ascending the hill, slates are 

 again found above it. On the opposite side of the road it forms a 

 bold rock, and also appears in the bottom of the valley, close to the 

 new church. Thin slices of the least altered portions of the rock 

 exhibit under the microscope numerous plates and crystals of augite 

 in various stages of alteration, many comparatively large crystals of 

 felspar (for the most part highly metamorphosed), grains of mag- 

 netite, and many long hexagonal needles of apatite. 



These original constituents are enclosed in a more or less clear 

 felspathic matrix, thickly crowded with minute green and pale brown 

 belonites, together with flakes of hornblende, which give the general 

 colour to the mass. 



The augite occurs in large grains of irregular shape, and also in 

 distinct crystals. The central parts are frequently quite unaltered* 

 but are usually surrounded by a border of fibrous green hornblende : 

 sometimes there is merely a central grain of augite remaining ; and 

 tihe^e are numerous examples of the completion of the process by its 



