354 rPvOE. J. w. JUDD on the propylites 



appears to have left all exact petrographical study of the materials till 

 after the completion of his field-work, and this led him to the erro-. 

 neons conclusion that the propylitic rocks were simply the plateau- 

 hasalts altered by eontact-metamorphism. Two dozen slices made 

 from specimens collected by Dr. Geikie were placed in the hands of 

 Dr. Hatch for microscopical study, and that gentleman's notes are 

 characterized by his usual accuracy and acumen. He noticed the ex- 

 treme alteration of the originally glassy base with the development 

 of secondary felspars, the complete change of the porphyri tic crystals, 

 and the development of chlorite, epidote, aud other secondary 

 minerals in the rock at the expense of the original constituents. 

 But Dr. Hatch's notes afford no support whatever to the idea that 

 these rocks are simply the plateau-basalts altered by eontact-meta- 

 morphism. 



B. Structure. 



That the rocks which in their altered form we now refer to the 

 propylites were, some of them, effusive (lavas and cupolas), while 

 others were intrusive (laccolites, sheets, and dj^kes) there cannot be 

 the smallest doubt. ]^o warrant can be found from the study of 

 these rocks for making a fundamental distinction of certain petro- 

 graphic types, as belonging to the effusive or volcanic series (Erguss- 

 gesteine), and others as belonging to the intrusive or Plutonic series 

 (Tiefengesteine). On the contrary, the central portions of some of 

 the very thick lava-currents poured out at the surface are more 

 highly crystalline than the rock of many eruptive masses. Still less 

 ground can I find, in this district, for instituting a class of " dyke- 

 rocks " ( " Ganggesteine ") ; for while dykes and veins are sometimes 

 found exhibiting very coarsely crystalline teiture, others finely 

 grained and even glassy may be seen side by side with them. 



C. Nature of Eoclc-masses. 



The rocks of the Western Isles of Scotland now referred to the 

 group of propylites are found exhibiting three kinds of relations 

 to the surrounding rock-masses : — 



1. Lava-streams. — That these rocks often constituted ordinary 

 lava-currents there cannot be the smallest doubt. Elat masses 

 covering considerable areas and presenting at their upper and under 

 surfaces the most strikingly scoriaceous and slaggy appearance 

 abound, and are found piled one upon another to the depth of many 

 hundreds or even thousands of feet. As a general rule, to which, 

 however, ihere are a few exceptions, the lava-currents composed of 

 porphyritic rocks are short and bulky, and they can seldom be 

 traced to a distance of many miles from their point of emission. In 

 this respect they present a most striking contrast to the olivine- 

 basalts, which, as a rule, must have been poured out as lavas of 

 great liquidity, and probably flowed distances of twenty, thirty, or 

 even forty miles from their points of emission. 



Professor J. D. Dana, in his suggestive memoirs on the Hawaiian 



