Mineralogy and Petrography. 453 
tion of its feldspar, locally passes into limburgite. Some of the 
larger of the feldspar crystals in this rock show the hour-glass 
structure frequently seen in augite. The rock of Pietzelstein is a 
nepheline basalt. Very detailed analyses of specimens of all these 
rocks are given, and each is very minutely described.——According 
to Harker,‘ most of the dyke-rocks of the Island Anglesey, off the 
northwest coast of Wales, are diabases and augite porphyrites. 
One of these dykes cutting a caleareous shale has converted this 
rock into a lydianite, in which calcite, clusters of garnet, and anal- 
` American Naturalist, 1885, p. 497. 
i Copper-Bearing Rocks. Monog. V., U. S. Geol. Survey. 
, Neues Jahrb. f. Minn., ete. 1888. I., p. 81. 
Geol. Magazine, 1887, Sept., p. 410, and Dec., p. 545. 
