184 Proceedings of Royal Society of Edinburgh, [april 4 , 
fication, and that originally these rocks contained no inconsiderable 
proportion of glassy matter, especially in their upper and under 
portions. In a number of the sections examined, however, no 
trace of devitrified matter was observed, the ground-mass in such 
cases consisting of an aggregate of microliths and minute crystals 
of plagioclase, closely felted together, hut containing interstitially 
abundant granules of ferrite, or magnetite passing into ferrite, along 
with minute granules of pale greenish-yellow, and yellow serpentin- 
ous matter and limonite, which are doubtless alteration products 
replacing hornblende or pyroxene, or both. The porphyritic in- 
gredients of these rocks are seldom prominent. Small and large 
lath-like crystals of plagioclase are not uncommon, the larger 
crystals seldom exceeding 1 mm. in length, and they are mostly 
broken. They sometimes show fine zonal structure. The larger 
crystals seem to he most common when the ground-mass is com- 
posed chiefly of devitrified matter, with very minute microliths. 
Pseudomorphs after hornblende and augite appear more or less 
plentifully. These consist partly of yellow or greenish-yellow 
serpeutine, sometimes veined with chrysolite, and partly of limonite. 
Not infrequently the pseudomorph is composed internally of ser- 
pentine, the outlines of the crystalline form being defined by 
limonite, or magnetite and limonite. Very often the limonite forms 
a meshwork of veins running through the serpentine, and occasion- 
ally these veins approximate in direction to the cleavage-planes of 
the original mineral. In many cases the shape of the pseudomorph 
gives one no hint as to whether the replaced mineral was augite or 
hornblende. Very often, however, the form is that of hornblende. 
This is specially the case with such pseudomorphs as have well- 
marked ferritic borders. In these one sees that the original 
mineral must have been broken, and more or less corroded, the 
ground-mass having often eaten into the heart of the crystal. A 
few pseudomorphs show very distinctly the form of augite, and 
some of these also contain inclusions of the ground-mass. Many of 
the pseudomorphs are quite amorphous, some composed almost 
entirely of serpentine, others almost exclusively of limonite : what 
proportion of these may represent hornblende, and what proportion 
augite, it is impossible to say. Of pseudomorphs having more or 
less definite forms, those after hornblende appear to be the most 
