251 



hornblende by alteration. The idiomorphic prisms show the clinopinacoid 

 (010) as well as the ordinary prism faces and lie in a matrix of felspar (see 

 Fig. 2, Plate XXIX). The ophitic masses are, on the other hand penetrated by 

 lath-shaped felspars in the same way as ophitic plates of augite (see Fig 1, 

 Plate XXIX). We have direct evidence, therefore, that in some of these rocks 

 the hornblende preceded, and in others succeeded the felspar. The 

 ophitic character is especially noticeable in those rocks which contain a large 

 amount of hornblende. All that has been said with reference to the 

 hornblende of the picrites (see ante p. 90) Avill apply to the hornblende 

 of these rocks. 



The plagioclase of the least altered rocks is perfectly fresh and shows 

 the characteristic twin lamellation. In the altered rocks, which are by far 

 the most numerous, turbid plagioclase occurs and the mineral is often 

 represented by pseudomorphs which have lost all individuality. Iron- 

 ores and apatite are invariably present and the latter mineral is often 

 very abundant. A nearly colourless pyroxene and pseudomorphs after 

 olivine may or may not be present. Calcite frequently occurs in consider- 

 able quantity especially in the olivine pseudomorphs. The highly altered 

 condition of many of the rocks often makes it very difficult to determine 

 their original characters. 



In texture the rocks vary from coarsely crystalline to compact. The 

 large ophitic plates of hornblende in the former are sometimes very 

 conspicuous and characterized by the lustre-mottling already referred to 

 in describing the picrites. The finely crystalline and compact rocks 

 resemble dolerites and basalts in general aspect. Rocks with idiomorphic 

 hornblende were observed by Mr. ALLPORT at Marston Jabet. They occur 

 also in the Tuttle Hill quarry, near Nuneaton. Allotriomorphic (ophitic) 

 hornblende is more common. It occurs in rocks from Atherstone Outwoods 

 Purley Park, Chilvers Coton and doubtless many other localities. Olivine- 

 augite-hornblende-plagioclase rocks occur in the neighbourhood of Ather- 

 stone and especially in Purley Park. Associated with the above hornblende- 

 bearing rocks we find others (e.g., specimens from the large quarry close 

 to one of the Nuneaton railway stations) which appear to be altered 

 plagioclase-pyroxene rocks (diabases). 



Plagioclase-hornblende rocks often containing a certain amount of 

 colourless pyroxene occur at Brazil Wood near Mount Sorrel in Leicester- 

 shire, at Little Knotts near Keswick, in the Lleyn Peninsula of 

 Caernarvonshire and in Anglesea. As already stated they pass into 

 hornblende-picrites by a decrease in the amount of felspar and an increase 

 in the amount of olivine. They are also intimately related to the dolerites 

 (diabases) and the hornblende-dolerites or -diabases may be regarded as the 

 connecting links. 



Dr. HEDDLE W refers to some remarkable rocks occuring in Banffshire. 

 A rock occurring west of the Battery at Portsoy is composed of labradorite 

 and hornblende together with a certain amount of ilmenite, sphene and a 



(1) Trans. Roy. Soc. Edin. Vol. 28. Chapters on the Mineralogy of Scotland. Parts 

 I. and II. 



