DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 77 



to be detected, even by green alteration products. Magnetite is sometimes present in 

 such quantity as to affect the compass of the field-geologist. Porphyritic varieties 

 occur with larger crystals of a different form from the laths of the base ; but such 

 varieties are, I think, less frequent among the plateau-rocks than among the dykes. 



In a few localities, there are found intercalated with the ordinary dark heavy 

 dolerites and basalts certain pale rocks of much lower specific gravity (2 7 1-2 7 4). 

 Externally these sheets are dull in texture, sometimes strongly amygdaloidal, sometimes 

 with a remarkable platy structure, which, in the process of weathering, causes them to 

 split up like stratified rocks. Examined with the microscope, they are found to consist 

 almost wholly of felspar in minute laths or microliths, but in none of my specimens 

 sufficiently definite for satisfactory determination. In one of the best slides Dr F. 

 Hatch, in whose hands I placed it, finds that " each lath of this abnormal felspar passes 

 imperceptibly into those adjacent to it ; the double refraction being very weak, and the 

 twin-striation, if present, not being traceable." He suggests whether the rock " originally 

 consolidated as a glass, poor in iron and magnesia, the development of the felspar being 

 due to devitrification." * Some of the varieties are amygdaloidal, the cells being filled 

 with epidote, which also appears in the fissures, and sometimes even as a constituent of 

 the rock. To such compounds I do not know that any existing petrographical name is 

 applicable. As they form the upper portion of Ben More, and the tops of some of its 

 neighbours in Mull, I have been in the habit of speaking of them as the " felspathic 

 lavas "or " pale-group " of Ben More, and it will be preferable to use some such vague 

 definition until their true chemical and mineralogical characters have been worked out. 



Passing now to the occurrence of the lavas as beds of rock in the plateaux, we find 

 them to present three well-marked types, all of which, however, pass into each other. 

 1st, Massive and amorphous ; 2d, Prismatic ; 3d, Amygdaloidal and slaggy. 



1. The more coarsely crystalline varieties (dolerites) are apt to occur in thick massive 

 beds, with no definite structure except the usual somewhat irregular joints placed 

 perpendicularly to the upper and under surfaces. In their general aspect, such beds 

 cannot readily be separated from intrusive sheets. But where they are not intrusive, 

 they will generally be found somewhat cellular towards their upper and lower surfaces ; 

 while, where intrusive, they are generally more close-grained there than anywhere else. 

 Rocks of this character are less frequent than those of the other two varieties. 



2. Prismatic structures are typical of the more compact heavy basalts. A considerable 

 variety is observable in the degree of perfection of their development. Where they are 



* In the course of my investigations I have had many hundreds of thin slices cut from the Tertiary volcanic rocks 

 for microscopic determination. These I have myself studied in so far as their microscopic structure appeared likely to 

 aid in the investigation of those larger questions of geological structure in which I was more specially interested. But 

 for their detailed examination I have placed them with Dr Hatch, in whose hands, together with the large series of 

 specimens accumulated by the Geological Survey, they will form the subject of a future memoir on the microscopic 

 petrography of this most interesting group of rocks. He has submitted to me the results of his preliminary examina- 

 tion, and where these offered points of geological import I have availed myself of them by citations in the course of 

 this memoir. Professor Judd, in a series of valuable papers, has discussed the general petrography of the Tertiary 

 volcanic rocks ; Quart. Jour. Geog. Soc, vols, xxxix., xli., xlii. 



VOL. XXXV. PART 2. L 



