342 PROF. J. W. JUDD ON THE PROPYLITES 



and character, and, except where influenced by surface-agencies, 

 seldom present any great signs of alteration ; they are indeed,, 

 as a rule, singularly fresh and unchanged in their appearance. 



But in the " felstones " which underlie the basalts the most ex- 

 treme metamorphism is seen to have taken place ; the felspars are 

 found to be so completely kaolinized that it is sometimes impossible to 

 decide whether they should be referred to orthoclase or to plagio- 

 clase ; the pyroxenes, amphiboles, and micas are converted into iso- 

 ropic mixtures or into minerals of the chlorite group ; while in 

 many cases the formation of epidotes and other secondary minerals 

 at the expense of the original constituents of the rock has gone on 

 to such an extent as to completely obliterate their distinctive charac- 

 ters. It may be safely asserted of many of these rocks that they 

 are at the present time entirely made up of secondary minerals, the 

 porphyritic constituents being represented by pseudomorphs, while 

 the ground-mass has been completely recrystallized. 



In 1874 I had beeu able to study only a few of the leading types 

 of these rocks microscopically, and consequently did not feel justified 

 in attempting a complete diagnosis of their varieties ; therefore I con- 

 tented myself with the grouping them under a convenient field- 

 name. 



During the last fifteen years, however, I have devoted much time 

 to the study of these rocks both in the field and in the laboratory. 

 The result of these studies is 'to show that among these "felstones " 

 there are presented to us many interesting types of rhyolites, dacites, 

 and sanidine-trachytes, intercalated with which are a few basalts, 

 some of them of the same ophitic type as those so abundantly poured 

 out at a somewhat later date. ■ With these there also occur certain 

 rocks which are so remarkable in their mineralogical constitution 

 and structure that they do not seem capable of being referred to 

 any of the accepted petrographical types ; and these anomalous 

 rocks I hope to describe on a future occasion. 



Eut the great majority of these Hebridean " felstones " prove to 

 belong to the family of the andesites, and to that " pathological 

 variety,''* to use Eosenbusch's expressive term, to which the name 

 of " propylite " has been given. 



The delay in publishing the results of .my researches upon these 

 curiously altered rocks has not been unattended with advantage. 

 Many interesting details concerning the propylitic rocks of other 

 areas have been published in the interval both in Europe and in 

 America : and during the same period there have appeared several 

 very important memoirs on the rocks of the Faroe Isles, Iceland, and 

 Greenland — districts in which the Tertiary igneous rocks present the 

 most marked analogies with those of our own Western Isles f. The 

 comparison of the much altered Scottish rocks with the very fresh 

 examples in these several districts has often afforded valuable aid 

 in the interpretation of the former. 



* Roseubusch, 'Massige Gesteine,' 2nd ed. p. 691. 

 t See Q. J. G. S. vol. xH. (1886) p. 53. 



