350 H. J. Loice — Rock Differentiation. 



in the camptonite variety, and by its only occasionally occurrius in 

 place of the so-called interstitial material of grades II and III, as 

 though only a partial change of that material into analcite had taken 

 place. It is considered to be a derivative from felspathic matter as 

 represented by Yan Hise, though all the appearances do not accord 

 with that supposition ; for lath felspars can be detected buried in the 

 analcite substance, and one instance is met with of analcite occurring 

 within a crystal of augite. It is interesting to note, in reference to 

 the relation of the analcite to the mass, that in the Inchcolm teschenites 

 it is described as " filling up the spaces between the other minerals as 

 if it were playing the part of groundmass." ^ 



It is an interesting coincidence to find that both the Inchcolm and 

 Lurcombe intrusions, which appear to be nearly related in constitution, 

 should also belong to the same geologic age, the Culm being of early 

 Carboniferous formation. Now if it could be shown that the analcite 

 of the Lurcombe mass is an original mineral, it would give to the 

 whole series of rock variations another bond of relationship. They 

 would then be considered as gradations in teschcnite types and 

 composition. The camptonite variety could be regarded, unless dis- 

 proved by analysis, as a purely hornblende teschenite with material 

 of analcite constitution, but of imperfect crj'stalline development, the 

 same material showing better crystal form in the true teschenites of 

 grades II and III, and appearing in still better condition in the purely 

 augite variety of grade IV. If, however, such a probable relationship 

 between the varieties was determined, the remarkable sequence of 

 changes and perfect gradations met with in the rock-mass would 

 demand some such hypothesis as gravitational differentiation to account 

 for its exceptional condition. 



EXPLANATION OF PLATE XIII. 



Fig. 1. — Grade I. Parts of three hornbleude crystals on N.E., S., and S.W. 



borders. Lath and micro-felspars in intervening cloudy groundmass. The 



light patch on N.N.E. border is a portion of a felspar plate which partly 



encloses the larger hornblende-needles of apatite and a black spot of 



pyrites S.E. x 25 diameters. 

 ,, 2. — Grade II. Hornbleude idiomorphic and attached to augite. Groundmass 



ot felspars, chlorite, and grains of calcite. Horublende in dark shade. In 



top right-hand quarter augite bordered and terminated by horublende. 



Light spots in crystals chlorite. Black pointed patch S.S.W. edge 



ihnenite. x 25 diameters. 

 ,, 3. — Grade III. Augite -with interstitial and peripheral hornblende. H. iu 



dark shade. Light patch at bottom chlorite, with calcite immediately 



above it. x 25 diameters. 

 ,, 4. — Grade IV. Augite (iu dark shade) ophitic to felspars. Clearest spots 



chlorite, x 50 diameters. 

 ,, 5. — Analcite in a crystal of augite. The bar ot white is analcite bordered and 



veined by calcite grains. The augite is much decomposed into calcite and 



chlorite, giving dark shades. The lightest patches are tresh augite. 



A part of a fringe of horublende is seen at the bottom of the figure where 



the boundary of the crystal is nearly reached, x 50 diameters. 

 ,, 6.— Felsparin sheaves of laths or skeleton form. All of dark shade is brown 



hornblende. Lightest patches calcite. Some of groundmass chlorite. 



A long needle of apatite to right, x 50 diameters. 



1 Op. cit., p. 127. 



