207 



NOTES ON NORTH OF ENGLAND ROCKS. 



i. 



ALFRED HARKER, M.A., E.G.S. 



The study of rocks in thin sections under the polarising microscope 

 is, comparatively speaking, a new branch of geology ; but it has 

 already opened out so large a field of research, and yielded results of 

 such interest, that it must be regarded as one of the most valuable 

 methods at our disposal for the solution of geological problems. 

 The great advances made on this line in the last twenty years are 

 mainly due to the labours of the German petrologists ; but, since 

 Dr. Sorby first led the way, there have not been wanting in England 

 zealous workers in this field, and the study is now becoming decidedly 

 more popular. 



The chief obstacles have been the want of a standard work on 

 the subject in our own language, and the absence of any guide for 

 the beginner to the petrology of districts in the British Isles. These 

 needs have been supplied to a great extent by recent literature, and 

 especially by the publication of Mr. Teall's 'British Petrography'; 

 but descriptions of typical rocks from definite and easily accessible 

 localities may still be of service to students of micro-petrology. The 

 Northern Counties of England furnish examples of a large variety of 

 interesting types, a few of which will be described below. We shall 

 require for most purposes only a microscope provided with a low- 

 power objective, movable polariser and analyser, and rotating stage. 

 It will be convenient to describe in most cases particular slides, but 

 each rock will be so selected that specimens from the same locality 

 will not be likely to show any important variation from that described, 

 (i) Porphyritic Granite or Gra?iite-porphyry of Shap jFc//s, West- 

 morland. — This rock, with its large pink felspar crystals, is well 

 known, both as an ornamental building-stone, and as the material of 

 the famous boulders which have been traced over Stainmoor and 

 across Yorkshire to the East coast. Since it has dark, but no white, 

 mica, it may be styled a Biotite-granite. 



Under the microscope a section shows a rock of the granitic 

 type of structure, consisting of felspars, quartz, biotite. sphene, and 

 magnetite. The felspars are of rather turbid appearance, and most 

 of them belong to the orthoclase species, either in simple crystals or 

 twins. These last are clearly shown in polarised light, between 

 'crossed Nicols,' by the two parts of the crystal giving different 

 polarisation-tints and extinguishing the light at different times as the 



July 1889. 



