726 EEPOKT — 1892. 



types. The paper concluded with a small list of Carboniferous fossils collected 

 in the same neifjhboiirhood, which have been described and figiuvd by Messrs. A. 

 H. Foord and G. J. Ilinde in the ' Geological Magazine ' for la'JO. 



4. On ForphyrUic Quart:: in Basic Tgneous lxocJ:s. 

 By Alfred Haeker, M.A., F.G.S} 



Basalts enclosing quartz-grains are known from the Tertiary volcanic areas of 

 the United States and from other parts of the world, and similar grains of quartz 

 occur widely, though sparingly, in the lamprophyre dykes (' mica-traps,' &c.) of 

 the North of England and other districts. Having regard chiefly to the latter 

 group of rocks, the author maintains that the quartz has not the characters of 

 mechanically derived fragments ; while it is dirticult, on the other hand, to conceive 

 of free silica crystallising from a basic magma. He supposes, as the source of the 

 rocks in question, a large magma-basin separated by <rravity into layers of different 

 density. The quartz crystallised out in the upper acid magma, and sank, owing to 

 its superior density, into the lower basic portion, which was afterwards separately 

 injected or ejected as a basic rock. 



5. On the Occurrence of Pisolitic Tuff in the Pevtlands. 

 By H. J. Johnstok-Lavis, M.B., Bac-cs-Sci., F.G.S. 



During the Saturday's excursion of the British Association to the Pentland 

 Hills, under the able guidance of Mr. Peach, we were shown some tuffs that cap the 

 ridge we were crossing from Logan Burn to Penicuik, or more correctly in the 

 third quarry down from the crest of the ridge between Carnethy and Scald Law. 

 These tuffs overlie scoriaceous andesites, which in their turn are superposed on 

 basalts. 



The tuffs are well stratified, at times finely banded, and altogether composed 

 of minute fragments, which in considerable part can be seen to be pumice. 



The interesting points about these tuffs, from a vulcanological point of view, is 

 the presence of pisolites, showing all the characters of those structures so common 

 in dusty tufts of recent formation — that is to say, with a coarse-grained nucleus 

 surrounded by a fine-grained shell. 



I originally believed these pisolites to be due to a sort of accretion, such as 

 forms in glazing pottery. They have, however, been shown to be produced by 

 drops of rain falling through a dusty atmosphere. Now such an origin will 

 explahi their structure — the rain drop will at first catch both coarse and fine 

 particles on its surface, but ]ireferably the larger ones, for they are more easily 

 wetted. As the drops of water get graduUy filled with solid particles the amount 

 of available moisture on the surface gets less and less, so that finally only the finest 

 powder can adhere. Pisolites frequently enclose a cavity in their centre, probably 

 due to the larger ])articles jamming together and forming a spherical vault around 

 the central drop of water. 



The deductions to be drawn from the existence of these pisolites ia that they 

 indicate that practically sub-aerial volcanoes, or at any rate vents in very shallow 

 water, are necessary for their production. 



I have recognised pisolites in tufts of Triassic age in Germany, but these of the 

 Old Red Sandstone period are the oldest recorded occurrence of such accretions, 

 and form another link of evidence in favour of the similarity between ancient and 

 actual volcanic action. 



' Published in extenso in the Geological Magazine, 1892. 



J 



