HOLE MALVEEN HILLS. 85 



them, exhibit that change in its crystalline condition (at the line of 

 contact with other rocks) which is produced hy more rapid cooling ; 

 nor does it give off any veins into adjacent rocks, all the granite-veins 

 of the hills being posterior to it in age. It lies in the jDlane of the 

 bedding of the other rocks, and not unfrequentlj its mica assumes 

 more or less of a linear arrangement, approaching that of gneiss. 

 This gneissoid structure is often better seen in the fresh surfaces of 

 (quarried blocks than in the rock in situ. Sometimes a belt of granite 

 is subdivided by narrow bands of gneissic rock into several beds, as 

 may be seen in a rock on the north side of the ravine above Lady 

 Huntingdon's Chapel, not far from the upper road *. 

 . Besides the common orthoclase or potash-felspar, there occurs in 

 many parts of the Malvern hills, especially in the rocks containing 

 hornblende, and in the syenite of the North Hill, another form of 

 felspar, which is either white or slightly tinged with yellow or pink, 

 and is usually more or less vitreous and translucent. It is less easily 

 cleaved than orthoclase, and sometimes exhibits striated surfaces like 

 oligoclase. This felspar has been found by the Rev. Mr. Timins to 

 contain too little silica for orthoclase or albite, and to be analogous 

 in its constitution to andesinef. The importance of this determina- 

 tion is greater than may at first sight appear. As this felspar occurs 

 in rocks which contain uncombined silica in the form of quartz, as well 

 as in those that do not, and as the ratio of its soda and lime, alumina, 

 and sihca is as 1 : 1 : 3, nearly, we are justified, I think, in concluding 

 that it could never have coexisted in a completely melted condi- 

 tion along with free silica also in a state of fusion, or it would have 

 entered into combination with the latter to form a felspar like albite 

 or orthoclase, in which the ratio is as 1 : 1 : 4 ; and we have thus con- 

 firmatory evidence that these rocks are not eruptive. 



Mr. Timins's researches also go to show that in some, if not all, 

 of the orthoclase-felspar, part of the potash is replaced by soda, lime, 

 and magnesia ; and that in the chemical constitution of the minerals 

 Avhich compose many of these rocks there is often a wide departure 

 from the theoretical formula — a fact quite consistent with their 

 metamorphic character $. 



* Although, therefore, the rock might sometimes be called, lithologically, a 

 granite, it is, petrologically, only an extreme form of gneiss. See also Horner 

 (op. cit. p. 285), -who calls attention to the difference in the appearance of the 

 Malvern granite from what he calls the " granite of Alpine countries." 



t The constitution of this felspar, as determined by Mr. Timins, at my 

 request, is as follows : — 



Silica 59-31 



Alumina 23-95 



Oxide of iron (determined as Fe^ Og) 2-66 



Oxide of manganese 0-40 



Oxide of copper 0-15 



Lime 3-66 



Magnesia 0-36 



Alkalies (determined as difference) ... 8-51 

 Water.,. 1-00 



100-00 

 X The Ecv. Mr. Timins's MS. 



