348 H. J. Lowe — Rock Differentiation. 



relation towards the larger felspars. In this instance the augite does 

 not appear in idioniorphic form, but is seen in plates with irregular 

 boundaries surrounding and enclosing some of the felspar ; while the 

 opposite change has taken place in the felspar, it occurring in crystal 

 form, though of only small dimensions. In this grade the interstitial 

 matter is mainly analcite with calcite and chlorite, the two latter 

 secondaiy products occurring also in the decomposing augite. In this 

 section analcite is seen enclosing lath felspars, and some glassy clear 

 grains, with apparent cleavage traces, may be secondary felspar. 

 Ilmenite and apatite continue as accessories. 



Grade lY is thus an instance of a rock, with the original 

 composition of a dolerite, that has undergone the changes which 

 constitute it a diabase, except for the presence of the mineral analcite. 

 This additional mineral presumably will necessitate the variety being 

 given either a specific name or a differentiating prenomen such as 

 analcite-diabase or augite-teschenite. 



Pieces of the same intrusive obtained from the output of the mining 

 operations on Stancombe farm show a rock more altered than any 

 from the Lurcombe exposure. This might be expected, as the fact of 

 a metalliferous vein being sought for there arose from alterations in 

 the appearance of the surface material, due doubtless to some local 

 changes or disturbances that were misinterpreted by the miners. A 

 hand-specimen of the igneous rock obtained from the mining debris 

 shows long grey crystals and spots of white in a dai'k-grey base. 

 Sections present neither hornblende nor augite. Both have entirely 

 disappeared and their places taken by paramorphs of an opaque 

 fibrous mineral threaded occasionally with chlorite. Chlorite and 

 calcite occur in other parts of the sections, as well as fairly well- 

 formed plates of a coloured mica, occurring sporadically, and most 

 frequently not fresh but considerably chloritized. The felspars are in 

 much the same condition as before described, and analcite has been 

 detected in only one section. The accessories ilmenite and apatite do 

 not appear to have suffered any change. 



Considering the varieties of rock described above as the products of 

 the same mass and of a single intrusion, the association of such 

 differences is so unusual that no general or current explanation of 

 rock differentiation seems to apply in this instance. As far as my 

 acquaintance with the literatui'e of igneous rocks extends, I am aware 

 of no similar or analogous case, and in endeavouring to find a solution 

 for what appears an extremely rare, if not unique, occurrence, I can 

 suggest no explanation which seems to so fully and simply account for 

 the facts as by considei'ing the variations due to a differentiation of 

 magmatic constitution by gravitation. It has been remarked that there 

 is no evidence in hand-specimen or section of dynamic deformation 

 within any part of the mass, and though not far from the granite it is 

 outside the marginal area influenced thereby. The camptonite variety 

 occurs at the top or upper side of the intrusion, and the evidence 

 points to the decrease and even disappearance of the hornblende as the 

 lower part of the mass is reached. This suggests the following 

 explanation : — Assume a homogeneous magma to be inti'uded between 

 horizontal strata. It is so constituted that the first important mineral 



