Vol. 50.] OF CAEEOCK FELL. 317 



little more acid. This zonary structure seems, among plutonic 

 rocks, to be specially characteristic of those which, without being 

 true acid rocks, have developed free silica as the last stage of con- 

 solidation. E-osenbusch has remarked the general absence of 

 zonary structure in the felspars of ordinary gabbros. 



The felspar is, as a rule, though not invariably, of earlier 

 crystallization than the pyroxene, and thus, while tbe latter mineral 

 never builds very characteristic ophitic plates, the structure of the 

 rock approaches that of a diabase. In view, however, of its coarse 

 texture and of the peculiarities of the pyroxenic constituent, I have 

 preferred to retain the name ' gabbro ' used by other writers. 



The dominant pyroxene, as remarked by Dr. Trechmann, is 

 certainly a monoelinic one. It rarely shows any crystal boundaries, 

 but occurs in allotriomorphic plates and wedges. The colour is of 

 the light greyish-brown tone so frequent in diallage and salite, and 

 there is no sensible pleochroism. Besides the well-marked prismatic 

 cleavage, there are occasional indications of others parallel to the 

 orthopinacoid and clinopinacoid. Simple twinning on the usual 

 law, parallel to the orthopinacoid, is not infrequent, but this rarely 

 gives rise to repeated lamellation. The most conspicuous feature of 

 the mineral is a very delicate lamellation parallel to the basal plane, 

 often marked by a certain amount of ' schillerization.' In a clino- 

 pinacoidal section the structure makes an angle of about 75° with 

 the cleavage-traces, and if the crystal be also twinned on the usual 

 law a very characteristic ' herring-bone ' appearance results. (See 

 PI. XVII. fig. 1.) This has been figured by Mr. Teall l from the 

 Whin Sill, a rock having several special features in common with the 

 Carrock Pell gabbros. The characters of our pyroxene thus connect 

 it with salite rather than with diallage ; but, as the analyses of the 

 rocks show that the mineral is rich in alumina, I shall speak of it as 

 augite. 



The most usual secondary alteration of the augite is -that which 

 results in a rather fibrous hornblende of a pale yellowish-green 

 tint. The change begins at the margin of a crystal, and spreads to 

 the interior, the augite and hornblende always having the usual 

 crystallographic relation to one another. The completed pseudo- 

 morph still shows the orthopinacoidal twinning and the basal 

 striation with its incipient schiller-structure. Small patches of 

 brown hornblende in the interior of the fresh pyroxene seem to 

 represent an original intergrowth, but these are of quite exceptional 

 occurrence. Brown biotite, apparently a highly ferriferous variety 

 comparable with haughtonite, is a frequent accessory constituent of 

 the gabbro, but seems to occur only in special circumstances, which 

 will be noticed below. 



Although the original identification of hypersthene seems to have 

 been erroneous, it is probable that a subordinate rhombic pyroxene 

 was originally present in parts of the gabbro. Such is the most 

 likely interpretation of certain pale green, fibrous pseudomorphs seen 



1 Quart. Journ. G-eol. Soc. vol. xl. (1884) p. 647, pi. xxix. fig. 2. 

 Q. J. G. S. No. 199. z 



