﻿part 2] 



LUBGECOMBE MILL LAMPEOPHYBE. 



79 



chlorite previously mentioned, are exclusively orthoclase. This 

 fact seems to indicate that they represent only the later stages of 

 felspar growth, and this theory is supported hy the zoning of the 

 felspars in the body of the rock. 



A fragment of the rhombohedral carbonate from one of the 

 vesicles was found to remain suspended in bromoform with a specific 

 gravity of 2 - 85 ; and as, in thin section, this mineral is not stained 

 after treatment with Lemberg's solution, it must be dolomite. 



The following analysis (I), made by Mr. J. H. Williams, of the 

 Scientific & Technical Department, Imperial Institute, may be 

 compared with those of the two minettes mentioned below. 



The comparatively small percentage of potash makes it un- 

 desirable to apply the name of minette to this rock; poverty 

 in silica and richness in alumina are also brought out by the com- 

 parison. Although the specimens from which the analysis was 

 made were some of the freshest obtainable, of the three rocks, the 

 analysis indicates that this is the most decomposed, 





I. 



II. 



III. 





Per cent. 



Per cent. 



Per cent. 



Si0 2 . 



44-53 



49-14 



50-98 



TiO., 



1-02 



1-81 



1-25 



Al.,(). t 



17-09 



14-89 



16-13 



FeA 



1-79 



1-08 



4-20 



FeO 



4-49 



3-88 



3-24 



MnO 



0-11 



0-14 



J0-17 



(CoNi)O 



trace 



0-08 



trace 



BaO 



0-49 



0-49 



0-20 



SrO 



trace 



nt. fd. 



trace 



CaO 



8-30 



513 



5-50 



MgO 



7-57 



7-07 



7-28 



K,0 



2-21 



5-82 



4-82 



Na,0 



2-70 



2-74 



2-99 



Li/) 





trace 



trace 



H.,0 at 105° C 



0-17 



0-15 



0-44 



K,0 above 105° C 



2-19 



1-16 



1-46 



P.,0 5 



0-64 



1-49 



0-74 



FeS 2 



0-64 



0-32 



0-43 



C0 2 



6-01 



4-94 



0-58 



F 



0-05 



0-20 





CI 



0-08 



0-05 



0-07 



Total 



100-08 



100-58 



100-48 



Less 0 for F & CI . 





o-io 



0-02 







100-48 



100-46 



I. Biotite-lainprophyre, Lurgecombe Mill, Ashburton (South DVvon). (Anal. 

 J. H. Williams.) 



II. Minette, railway- cutting near Lemail Farm, 1 mile west of St. Mabyn 

 Church, Cornwall. (Anal. E. G. Eadley.) ' The Geology of the Country 

 around Padstow & Camelford' Mem. Geol. Surv. 1910, p. 62. 

 III. Minette, Gannel Quarry, Pentire, Newquay. (Anal. W. Pollard.) ' The 

 Geology of the Country near Newquay ' Mem. Geol. Surv. 1906, p. 61. 



H2 



