Alfred Sarker — Various Crystalline Rocks. 169 



and their formation into slates, was the result of very severe 

 "dynaniometamorpbic" action and could not be brought about, 

 as regards the coarser parts of the deposits, without its aid. Still, 

 these are only "views," and nobody's views are of very much 

 authority in the present state of our knowledge of these matters. 

 "We want, perhaps, more facts and less views. My excuse for stating 

 mine above must be my desire to escape some remorse for the thing 

 Dr. Irving says I have done ! 



I may mention that since my first paper I have examined clays 

 and shales from the Derbyshire coal-field (Clay Cross), and with the 

 same results. Materials from all the coal-fields seem to be of like 

 nature, derived from the same or a similar source, — a granite (or 

 gneiss ?) with two micas, biotite being in excess. 



As regards the composition of all these fireclays, I would once 

 more emphasize the fact that they are not " hydrous silicates of 

 alumina." The results of chemical analysis chance to lend them- 

 selves tolerably easily to this interpretation ; but it is quite a wrong 

 one. The analyses are quite as well suited to a material mainly 

 composed of strongly hydrated altered micas. Writers dealing with 

 fireclays seem to have thought that every chemical analysis must be 

 worked up into a " formula," just as is done so much in the case of 

 metallurgical slags, even though in both cases a glimpse through the 

 microscope would show the complexity of the substance and the 

 ■uselessness of the formula. 



WOODWARDIAN MuSBUM NoTES. 



IV. — On Vakious Crystalline Eooks. 



By Alfred Barker, M.A., F.G.S., 



Fellow of St. John's College, Cambridge. 



(i.) Pyroxenite (Wehsterite) from Fobello, Lomhardy. 



CRYSTALLINE rocks containing neither felspar nor olivine have 

 received but little or no attention, except from one or two 

 American geologists. Dr. G. H. Williams^ has recently drawn 

 attention to these rocks, of which he proposes to constitute a family 

 of Pyroxenites coordinate with Eosenbusch's family of Peridotites ; 

 and has described from Maryland and North Carolina a type consist- 

 ing entirely of monoclinic and rhombic pyroxenes, to which he gives 

 the name Websterite. The occurrence of a similar rock in Northern 

 Italy seems worth recording. Our specimens were collected by Mr. 

 E. J. Garwood. 



To the eye the rock appears as a coarsely crystalline aggregate of 

 lustrous black diallage, in which the rhombic pyroxene which forms 

 the second constituent is scarcely to be distinguished. The specific 

 gravity of the rock is 3-229. 



A slice [1086] shows the rock to consist of large plates of diallage 

 moulding smaller grains of bypersthene, neither mineral having 

 any external crystal form. 



The hypersthene is the first-formed mineral, and builds irregular 

 1 American Geologist, July 1890. 



