AN]SriVEESARY ADDEESS OF THE PEESIDENT. 1 29 



position that it may be, though doubtfully, included among the 

 bedded basalts. This is the quarry near Blackburn, to the east of 

 Bathgate, where I originally observed it.^ The rock occurs there on 

 the line of the basalt-flows from the Bathgate Hills, and I mapped 

 it as one of them before the microscope revealed the remarkable 

 composition of the mass. I am still inclined so to regard it, though 

 I cannot prove that it may not be a sill. 



The microscopic structure of this rock is now well known. As 

 exposed in the quarry, an interesting difference is observable between 

 the lower and upper parts of the sheet. The lower portion is a 

 picrite with abundant serpentinized olivine, large crystals of augite, 

 and a considerable amount of ores. The upper portion, on the other 

 hand, has plagioclase as its most abundant definite mineral, with a 

 minor quantity of minute prisms of augite and of iron ores, and 

 scattered crystals of olivine. Here, within the compass of a few 

 yards and in one continuous mass of rock, we have a transition from 

 a variety of olivine -basalt into a picrite. 



2. By far the largest number o:l the lava-beds of the puy-type 

 are Basalts. They exhibit some variety of structure as seen in the 

 field. Some are solid, compact, black rocks, not infrequently 

 columnar and weathering into spheroidal exfoliating forms. Others 

 are somewhat granular in texture, acquiring green and brown tints 

 by weathering, not infrequently showing amygdaloidal kernels and 

 even passing into well-marked amygdaloids. Many of them exhibit 

 a slaggy structure at their upper and under surfaces. 



These external differences are an index to corresponding variations 

 in composition and microscopic structure. The rocks may be 

 arranged in two groups, in one of which there is no olivine, while 

 in the other that mineral is present. Dr. Hatch, in whose hands 

 I placed a series of illustrative slides prepared from the rocks of the 

 basin of the Firth of Forth, and who I hope will ere long be able to 

 undertake an exhaustive study of the petrography of these rocks, 

 informs me that he can recognize the following distinct types of 

 structure and composition. 



A. Basalts with Olivine. — (a) Containing large porphyritic oli- 

 vines, usually more or less serpentinized, granular augite, scattered 

 grains of magnetite, and only a small amount of felspar micro- 

 lites, thus approaching picrite. Lavas of this character occur at 

 the bottom of the volcanic series of the Bathgate Hills at Hillhouse 



1 Trans. Eoy. Soc. Edin. vol. xxix. (1879) p. 50G. 



