188 



products about the relations of which there is some difference of opinion. 

 Dr. GEIKIE (i> holds that we have here the record of two district series of 

 volcanic eruptions ; the one belonging to the Calciferous Sandstone, the 

 other to a much later period, probably the Permian. Prof. JUDD, on 

 the other hand, maintains that both sets of volcanic products belong 

 approximately to the same period, viz., to the Lower Carboniferous/ 2 ) 



The rocks of the Long Row, Haggis Knowe and St. Anthony's Chapel 

 belong to the earlier series. They are finely crystalline rocks of porphyritic 

 but not conspicuously porphyritic texture. The porphyritic constituents 

 are felspar, olivine and augite. The ground-mass is composed of lath- 

 shaped plagioclase, magnetite and minute granules of augite. Serpentine 

 and chlorite are abundant in certain specimens. The rock was originally 

 an oli vine-basalt. Fluxion structure is sometimes well shown in the 

 arrangement of the minute plagioclase crystals. 



One of the most remarkable rocks of the district is the olivine- 

 basalt of the Lion's Haunch. It belongs to the later series. It is a 

 black rock which is rendered conspicuously porphyritic by crystals of 

 augite, felspar and olivine. The augite crystals show the common forms 

 (see Fig. 1, p. 157) and are frequently well exposed on a weathered 

 surface of the rock. They often measure an eighth of an inch or more 

 in length. The felspar (probably bytownite) occurs in thick tabular 

 crystals. Sections exposed on a fractured surface are often a quarter 

 of an inch in length, and show well marked striation on the basal 

 cleavage. The lustre of the cleavage surfaces is often interrupted by 

 the presence of inclusions. Olivine is very abundant and wonderfully 

 fresh. It appears as yellowish green oily looking spots on a freshly 

 fractured surface and may be readily detected without the use of the 

 lens. All the porphyritic constituents of this beautiful rock may be 

 determined without even the use of the pocket lens. Thin sections show 

 that the above constituents lie in a ground-mass composed of minute 

 granules and crystals of augite, crystals and grains of magnetite and 

 colourless felspars which sometimes give lath-shaped sections, and some- 

 times appear as irregular plates. Felspar appears to play the role of 

 matrix in the ground-mass and amorphous interstitial matter is rare, 

 or present only in very small quantity. The porphyritic olivine crystals 

 illustrate all stages in the process of serpentinisation, but are generally 

 very fresh. 



The rock on the east side of Dunsapie Loch, mapped as an inter- 

 bedded lava of Lower Carboniferous age, bears many points of resem- 

 blance to that just described. The same minerals occur in it in very 

 much the same relations but in different proportions. The porphyritic 

 elements are often so numerous as to give the rock a coarsely crystal- 

 line appearance. This and the comparative rarity of olivine are the two 

 principal characters which distinguish it from that just described. Under 

 the microscope porphyritic felspars are seen to be always repeatedly 



(1) Geology of Edinburgh. 



(2) Q.J.G.S. Vol. XXXI., 1875, p. 131. 



