W. M. Sutchings — Ask-slates of the Lahe-Didrict. 155 



A full understanding of the changes these deposits have under- 

 gone, and their present nature, would require a large amount of 

 fuller study, especially in a chemical direction ; but some of the 

 results of my observations may be worth recording as far as they 

 go, even if only on the chance that they may perhaps induce some 

 worker, with more time and opportunity at his disposal than I 

 possess, to take up the investigation of these rocks. In the Lake 

 District we have a wonderful field for the petrological microscopist 

 and chemist, which has been singularly neglected, I think, consider- 

 ing the avidity with which petrological work has been taken up of 

 late years. 



Perhaps this comparative neglect may be less seen in future, since 

 the paper by Messrs. Harker and Marr on the Shap rocks has shown 

 what splendid work may be done there by competent workers. 



If we examine a series of the finer-grained rocks of the district, 

 taking some of those worked as roofing-slates as especially typical, 

 we find considerable variation in texture, corresponding to differences 

 in composition as shown by the microscope. Thus the larger portion 

 of such slates are seen, in thin sections under low and moderate 

 powers, to consist of more or less well-defined and recognizable 

 fragments and grains of various kinds, — lapilli, felspars, calcite, 

 chlorite, etc., — bedded in and surrounded by varying amounts of a 

 paste, or base, so to speak, which is exceedingly fine-grained. This 

 very fine-grained material increases in some slates till it becomes 

 the sole constituent, while in others it plays only a more subordinate 

 part ; but it is always present and always, within certain limits, 

 similar in appearance under the microscope, till we pass over into 

 the coarser-grained ashes and tuffs which are not classed as slates 

 at all. 



In order to study the nature of this pervading " base," I will 

 select one or two special occurrences, in which the original nature 

 of the materials from which they have been formed can be ascertained 

 with considerable certainty. 



There is an old quarry in the valley of Mosedale, near Shap, in 

 which beautiful examples of these ash-slates occur. Bands of 

 various coarseness of grain succeed one another in narrow limits 

 and pass abruptly into one another, corresponding, apparently, to 

 sudden changes in the condition of the volcanic detritus being 

 deposited. A coarser band, for instance, will consist very largely 

 of good-sized lapilli, sharply outlined and well preserved, formed 

 for the most part of normal andesitic rocks of the district, but with 

 rhyolite also represented, and will contain also many detached 

 crystals and fragments of felspar, with varying amounts of grains 

 of calcite and patches of chlorite. 



A succeeding fine band will show none of these lapilli, recogniz- 

 able fragments of felspar, etc., but consists absolutely of the very 

 fine base which surrounds the lapilli, etc., in the coarser band. We 

 cannot doubt that it represents the alteration-product of the very 

 finest dust of the same materials as make up the coarser band, and 

 therefore that it consisted originally mainly of andesitic and felspathic 

 powder with a little rhyolite intermixed. 



