132 ME. J. MOBBISOK" OX [vol. Ixxiv, 



the past to calcite and to dolomite. To determine their true 

 nature in these rocks micro-chemical methods were adopted. 

 Lemberg's method was used for the contents of the vesicles, and 

 the result in every case proved the carbonate to be calcite and 

 not dolomite. The disseminated carbonates, however, remained un- 

 affected, consequently further treatment was necessary. For these 

 Heger's method was adopted, consisting of treatment with dilute 

 hydrochloric acid and potassium ferrocyanide, which depends for 

 its success on the presence of iron- — a condition easily satisfied in 

 these rocks. The carbonates throughout the sections so treated 

 were stained a deep blue, thus proving also to be calcite. Sections 

 of lamprophyres from various parts of the area were likewise 

 tested, but the result never varied. It is clear, therefore, that, 

 although the presence of a certain amount of magnesia is not 

 precluded (microscopic evidence indeed suggesting its presence), 

 the carbonates are predominantly calcic. 



The intrusions are found both north and south of the granite, 

 but are more common on the south. Those on the north are richer 

 in xenocrysts frequently of maximum size, a circumstance which, 

 taken in conjunction with the more acid nature of the ground- 

 mass, suggests derivation from the upper levels of the magma : 

 this is a point not devoid of significance. The specific gravity 

 varies from 2*642 to 2*732, indicating a wider range and a greater 

 density than the Potter-Fell rocks, although the presence of the 

 xenocrysts appreciably modifies the true magmatic density. 



The orthoclase-xenocrysts reach their maximum abundance in a 

 narrow dyke breaking through the andesitic ashes in the bed of the 

 Lowther, about a mile to the north of the granite. Thev exceed 

 an inch in length, but are smaller generally than those of the 

 granite, and are associated with quartz-grains, pale-green striated 

 felspars, and plates of mica, the compact grey matrix being quite 

 subordinate. Viewed in thin section, the orthoclases in form and 

 inclusions are comparable with those of the acid rocks ; while the 

 quartz-grains are rounded when small, but give indication of 

 hexagonal contours in the larger individuals. Well-developed 

 oligoclase Avith small angles of extinction is the most important 

 phenocryst. though occasionally an approach to andesine is notice- 

 able, and biotite is subordinate. The ground-mass is a very finely 

 micrographic intergrowth of orthoclase and quartz, with some 

 oligoclase. 



A dyke nearer the granite shows a slight advance in basicity, 

 not only in the increased importance of biotite as a phenocryst, 

 but more especially in the appearance of a second generation in 

 the ground-mass. The most interesting feature of the rock, how- 

 ever, is a basic segregation near the margin. It is more or less 

 identical with those of the granite, the chief constituents being 

 biotite, basic oligoclase, and sphene, with very little orthoclase 

 or quartz, and abundant needles of apatite. Associated with 

 the biotite and sometimes isolated is amphibole, appearing here 



