W. M. Hutchings—A Contact- Rock from SJiap. 315 



■knowledge whatever of tlie district round the granite ; and from its 

 character, macroscopic and microscopic, I set it down as an altered 

 sedimentary, probably a slate, with little or no quartz. Mr. Marr, 

 however, informed me that it is an altered ash, and it was sub- 

 sequently so described by him and Mr. Harker. 



It is so highly altered and completely regenerated that not a 

 particle of original material is left, and so far as microscopic 

 examination went, it was mainly on the strength of the presence of 

 certain new minerals, usually connected with altered sedimentary 

 rocks, that the erroneous inference was drawn which was corrected 

 by Mr. Marr's minute knowledge of the field-relationships of the beds. 

 The most striking point in the mineralogical constitution of the 

 rock is that it contains a considerable amount of both sillimanite 

 and andalusite, the specimens in question being the only ones 

 collected, either by Harker and Marr or by myself, which show 

 the presence of either of these minerals in an altered rock at Shap. 

 Tliere is nothing of note about the abundant andalusite, but the 

 sillimanite, in addition to bundles of very slender needles and fibres, 

 forms some rather unusually fine prismatic crystals. 



As has been previously pointed out, this is the only^ recorded 

 instance of the formation of sillimanite and andalusite in altered 

 volcanic material, and it is so far deserving of notice as being an 

 additional proof that the chemical composition of the rocks invaded, 

 and not their mode of origin or their mineralogical constitution, 

 influences the nature of the new minerals formed under the effects 

 of contact-action. 



Other minerals present are magnetite, apatite, a good deal of 

 perfectly colourless garnet, in rounded and irregular grains without 

 any crystal faces, a little white mica, and a still smaller amount of 

 biotite, both of the latter irregularly diffused in small patches. 



By far the main portion of the rock is made up of felspar, in 

 a mosaic of very varying degrees of development as to size of grain 

 and distinctness of individuals. It is beyond all question completely 

 a new formation, and among all the slides of contact-rocks which 

 I have studied I have never seen so complete and striking a develop- 

 ment of the mineral, due entirely to contact-action, as in this case. 



A large part of it is very fine-grained and acts as a sort of ground- 

 mass. In this occur many larger individual grains of the felspar, 

 easily identified as such, and several patches in each slide made up 

 of a mosaic of interlocking grains of strikingly larger dimensions. 



Quartz is either absent or very sparingly represented. If present, 

 it is only in the most fine-grained part of the mosaics, as it cannot 

 be identified in the coarser parts. 



Many of the felspar-grains, especially in very thin slides, show 

 a very fine lamination or striation, best seen in polarized light. In 

 smaller grains it makes more the effect of the minute striation which 

 has been recognized as frequently characterizing contact-felspar, but 

 in the larger grains of the rock it is often developed in a very much 

 higher degree, there being also various intermediate stages. Taking 

 the larger grains only, this lamellation in polarized light may be seen 



