DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



167 



On the south side of Beinn na Cro, highly indurated black and grey Lias shales and 

 sandstones have been tilted up steeply and indurated by the eruptive rock of the hill ; 

 and at one place some 800 feet above the sea, a little patch of altered basalt, lying on 

 the shale but close up against the steep declivity of granophyre, forms a conspicuous 

 prominence on the otherwise featureless slope. 



I have reserved for the present section of this memoir a fuller account of the meta- 

 morphism of the basalts, to which frequent allusion has been made as one of the evidences 

 of the posteriority of the eruptive bosses of rock round which it occurs. The field- 

 geologist observes that the basalts as they are traced towards these bosses lose their 

 usual external character. They no longer weather into spheroidal blocks with a rich 

 brown loam, but project in much jointed crags, and their hard rugged surface shows when 

 broken a thin white crust, beneath which the rock appears black, dull, and splintery. 

 They are generally veined with minute threads or strings of calcite, epidote, and quartz, 

 which form a yellowish-brown network that projects above the rest of the weathered 

 surface. Where they are amygdaloid al the kernels no longer decay away or drop out, 



Fig. 55. — Section at north end of Beinn na Cro, Skye. a, basalt, dolerite, andgabbro; b, granophyre of Beinn na Cro; 



ft 1 , dyke of granophyre ; cc, basalt dykes. 



leaving the empty smooth-surfaced cells, but remain as if they graduated into the 

 surrounding rock by an interlacing of their crystalline constituents. They then look at 

 a distance more like spots of decoloration, and even when seen close at hand would 

 hardly at first betray their real nature. 



From the specimens collected by me in Skye, Mull, and Rum, I have selected two 

 dozen which seemed to be fairly typical of these altered rocks, and have placed thin 

 slices of them for microscopic examination in Dr Hatch's hands. His notes may be 

 condensed into the following summary. One of the most frequent features in the slides 

 is the tendency in the component minerals to assume granular forms. In one specimen 

 from Loch Spelve, Mull, the rock, probably originally a dolerite, shows only a few 

 isolated recognisable crystals of plagioclase and augite, the whole of the rest of the rock 

 consisting of roundish granules embedded in a felspathic matrix. The felspar crystals 

 are sometimes broken up into a mosaic, though retaining their external contours. 

 Besides the granules, which are no doubt augite, a few grains of magnetite are scattered 

 through the rock, aggregated here and there into little groups. In another specimen, 



