Rambles among the Channel Islands / by a Naturalist.

Date:
[1855?]
Catalogue details

Licence: Public Domain Mark

Credit: Rambles among the Channel Islands / by a Naturalist. Source: Wellcome Collection.

    144/402 (page 132)
    of variable proportions of the minerals called felspar, quartz, and mica, and these are inti- mately blended together. Felspar is usually the predominating rock. Of the mixtures of these three ingredients in varying relative quan- tities, the granite of these islands, properly so called, is composed. From this variation in the relative proportion of its ingredients arises the difference in the brilliancy, texture, and colour of the rock. These minerals are generally united in what is termed a confused crystallization; that is to say, there is no regular arrangement of the crystals in ordinary granite; but a singular variety is sometimes met with, and exists in the rocks of the northern coast of Guernsey, which is termed, from its remarkable appearance, graphic granite. It is found chiefly in veins.* It is a compound of felspar and ; quartz, so arranged as to produce an imperfect j laminar structure. The crystals of felspar : appear to have been first formed, leaving be- tween them the space now occupied by the : ‘ darker-coloured quartz. This curious mineral i presents nothing unusual when a section is made ; perpendicularly ; but, when cut at right angles ^ to the alternate plates of felspar and quartz, it J presents broken lines which bear a singular n resemblance to Hebrew characters. • Lyell, Elements of Geology.