17-2 



be relied upon as a test of chronological relations except in limited areas. 



They occur in large eruptive masses, often forming mountains two or 

 three thousand feet in height, and from these masses apophyses run out in 

 different directions. The texture of the rocks is thoroughly granitic ; no trace 

 of amorphous matter is ever found. The constituents of the rocks are 

 plagioclase, augite, diallage, pseudo-hypersthene, enstatite, olivine, biotite, 

 and magnetite. These constituents are present in very different proportions 

 in the different varieties. Plagioclase, monoclinic pyroxene and olivine are 

 the principal constituents. Enstatite and biotite occur locally in certain 

 varieties. The magnetite of the gabbros occurs in cracks and as inclusions in 

 the olivine. It is never, according to Professor JUDD, a direct product of 

 the consolidation of the magma. Sometimes the olivines are rendered quite 

 opaque by the large amount of secondary magnetite which has been 

 developed. 



As a general rule and having regard to the whole district, felspar is 

 the most abundant and olivine the least abundant constituent. Cases 

 frequently arise, however, in which the olivine is more abundant than the 

 augite or diallage. 



Nodules and contemporaneous veins are common in certain portions of 

 the gabbro-masses, especially in the island of Rum. They are for the 

 most part binary compounds of the essential minerals of the gabbros, but in 

 some instances one mineral forms almost the entire mass of the nodule or 

 vein. As a general rule the gabbros are granular in texture. In Skye and 

 Ardnamurchan, however, porphyritic varieties containing felspar crystals, 

 two inches in length, are occasionally found. These large crystals appear to 

 have been formed before the consolidation of the ground-mass. They have 

 sometimes been cracked and the pyroxenic material has penetrated and 

 crystallised in the cracks. In the Isle of Rum the gabbros containing 

 plagioclase, pyroxene and olivine are found in intimate association with 

 troctolites, eucrites, picrites, and peridotites. This association is of great 

 interest, because it is found to be an almost constant feature in all the great 

 gabbro- areas. 



All writers on the Scotch and Irish gabbros of Tertiary age who have 

 studied the rocks with the microscope call attention to the frequent presence 

 of parallel rows of minute rods or plates in each of the three principal 

 minerals. As the nature and relations of these inclusions have been already 

 discussed (see ante, p. 28), it is unnecessary to do more than refer to the 

 subject in the present connection. 



The occurrence of a rock belonging to the gabbro-family (troctolite) in 

 the parish of Belhelvie, Aberdeenshire, has been noted by Professor 

 BONNEY. (1) It occurs in association with serpentine. The troctolite is a 

 moderately coarse-grained mixture of purplish-grey or whitish felspar, and 

 dull dark-green serpentine. Coarser varieties occasionally occur ; usually 

 the felspar is to the serpentine in about the proportion of three to two, but 

 occasionally a fragment is seen in which the former mineral distinctly 



(1) Ou bastite-serpentine and troctolite in Aberdeenshire. (J.M,, 1885, p. 439, 



