W. M. Sutchings — On some Lake-District Rocks. 543 



mention. It is a grey-green, highly vesicular rock, the vesicles 

 now filled with chlorite and chalcedony. In thin sections the 

 difference alluded to is apparent in the felspars of the ground-mass. 

 These to a very large extent consist of untwinned laths, many very 

 elongated in proportion to breadth, and extinguish mainly quite 

 parallel to their length. Such of them as are twinned are almost 

 without exception binary twins, and of these a large proportion 

 extinguish simultaneously and again quite parallel, or at very low 

 angles. There are a good many square tabular sections also seen, 

 more than in any other local andesites examined. The laths are 

 of various sizes, and pass down to very small ones with mainly 

 forked and ragged ends. So far as optic discrimination in thin 

 sections is a guide, the felspar in this ground-mass seems to be 

 mainly, if not almost wholly, orthoclase. There is a good deal of 

 devitrified interstitial glass and the usual amount of diffused 

 alteration-products. The porphyritic felspars are still fairlj'^ fresh, 

 and several of them in some of the slides examined appear to be 

 orthoclase, while in others they are wholly plagioclase, the ground- 

 mass remaining as above described. In one hand-specimen there 

 are large groups of felspar crystals clustered together in the manner 

 described by Mr. Teall in the case of the Tynemouth Dyke ; that is, 

 they are idiomorpJiic towards the outside of the group, and allotrio- 

 morphic inside it. Some of these groups are so large that they give 

 the effect, on fractured surfaces of the rock, of being infillings of 

 good large vesicles with felspar, but the microscope shows their 

 true nature. 



Some of this rock was prepared for analysis by crushing moderately 

 small and picking out sufficient bits quite free from vesicles or their 

 contents. The analysis was kindly made for me by Dr. J. B. Cohen, 

 of Owens College, and is calculated on dehydrated rock : — 



98-99 



These figures would stand equally well for a trachyte or an 

 andesite, the proportions of the alkalies being much the same as in 

 many analyses given of trachytes ; agreeing for instance almost 

 completely with one of the " typical analyses " given by Professor 

 Cole in his " Aids in Practical Geology." It appears that whether 

 a magma of certain composition will consolidate as trachyte or as 

 andesite, depends largely on conditions other than purely chemical. 

 The rock in question is not a trachyte, but it would become one by 

 a very moderate degree of further differentiation from the accom- 

 panying andesites. Eocks with a closely similar orthoclase ground- 



