﻿588 ME. J. V. ELSDEN ON THE IGNEOUS KOCKS [Aug. I905, 



warrant the placing of this rock among the monzonites ; but, as the 

 felspars are rather altered, only surface-exposures being accessible, 

 it is possible that some alkali has disappeared. From a general 

 point of view, I think it may be stated that the rocks in this 

 ridge vary from quartz-enstatite diorite towards 

 quartz-enstatite monzonite, and typical examples of the 

 latter rock might be expected to occur locally. There is certainly 

 a marked difference between the intrusions along this ridge and 

 the biotite-norites of Cam Llidi. The prominence of hornblende 

 also differentiates them from the rocks of St. David's Head, in 

 which hornblende seems to occur only sparingly towards the 

 eastern extremity. Rhombic pyroxene, however, continues to be 

 a characteristic and abundant component of the rock. 



We come next to Penberry Hill, which is an isolated boss-like 

 intrusion, lying in direct line with, and about a mile to the north-east 

 of, the Carn-Trelwyd ridge. Under the microscope this rock bears 

 some resemblance to that of Cam Trelwyd, but it is evidently of a 

 more basic type, containing less quartz and orthoclase, and only a 

 very insignificant amount of hornblende. The feature of this rock 

 is the predominance of a bronzite-form of enstatite, which occurs in 

 very numerous rectangular sections, with ill-defined terminations, 

 sometimes showing rather indistinct blunt domes at one end. It is 

 fibrous and slightly pleochroic, and seems to belong to an early 

 period of consolidation — preceding, in fact, all the constituents 

 except apatite (which is fairly plentiful) and some of the ilmenite. 

 It closely resembles the enstatites above described, and has a 

 tendency to enclose sphene-granules. The next mineral to crystal- 

 lize was felspar, which seems to be a not very-basic plagioclase, but is 

 rather too turbid for accurate determination. This was followed 

 by a pale augite in subordinate amount, sometimes twinned and 

 rather granular. There seems to be some orthoclase, with occasional 

 Carlsbad twinning ; and quartz with a very little micropegmatite 

 were the last minerals to crystallize. (See PI. XXXIX, fig. 3.) 



I made a chemical analysis of this rock with the following results : — 



Molecular 



Percentage, proportion. 



Silica 5442' '902 



Titanic acid 072 -009 



Alumina 15-34 '151 



Iron-sesquioxide , 067 '004 



Iron-monoxide 547 "071 



Manganese-oxide trace 



Lime 8-30 448 



Magnesia 669 466 



Potash 1-01 -010 



Soda 4-21 -068 



Phosphoric acid 0'57 '004 



Water expelled at 1 10° Centigr 047 



Water (combined) 260 



Carbon- dioxide trace 



10047 



