221 



with alkali derived from the eruptive rock. Thus, at Bochtenbeck, by 

 Niedersfeld, the unaltered rock contains 116 p.c. of soda, the altered 

 rocks. 4*94, 4'56 and 3'59 p.c. ; at Kuhlenberg, by Silbach, the unaltered 

 rock contains 115, the altered rock, 714 p.c. ; at Silberberg, the unaltered 

 rock contains 0'50, the altered rock, 6'03 p.c. We are thus led to the 

 important conclusion that the albite of the rocks altered by diabase is 

 largely if not wholly a secondary product due to the actual impregnation 

 of the surrounding sediment by material derived from the eruptive rock. 

 Similar instances of impregnation may occasionally be found in the rocks 

 surrounding granite-masses. 



So far reference has been principally made to the granular diabases. 

 In some localities we find rocks of similar mineralogical composition con- 

 taining conspicuous porphyritic crystals of plagioclase. These are far less 

 common than the granular diabases. Mr. TAWNEY described a good ex- 

 ample of this type,' 11 from the hill designated as a Trigonometrical station 

 one and a-half mile N.N.E. of Pwllheli. He terms the rock a diabase- 

 porphyrite. It is characterized by porphyritic felspars, up to a quarter inch 

 in length, embedded in a dark green ground-mass. Under the microscope 

 the felspars are seen to be much decomposed. They are represented partly 

 by diverging zeolite-needles and partly by indistinct granular matter. The 

 ground-mass contains augite in crystals and granular aggregates, ilmenite 

 and pseudo-amygdules containing chlorite and other substances. 



The basic rocks of the Lleyn peninsula and Anglesea frequently con- 

 tain hornblende and olivine. By the increase in one or other of these 

 constituents the diabases pass into hornblende-diabases, diorites, and horn- 

 blende-picrites. 



Compact augitic greenstones with interstitial matter (melaphyres or 

 altered basalts) are comparatively rare in North Wales. At least they 

 have not as yet received much attention if they occur on any extensive 

 scale. Such rocks occur near the top of Cader Idris. Under the micro- 

 scope one recognizes minute, lath-shaped sections of felspar, granules of 

 augite and dirty brown interstitial matter. Rocks of this type are some- 

 times vesicular and thus possess all the petrographical characters of lava 

 flows. 



A consideration of the facts given above only serves to show how ex- 

 tremely meagre our information as to the basic plagioclase-augite rocks of 

 North Wales really is. The most important points which have been estab- 

 lished with regard to them are : 



(1) The existence of a granular ophitic diabase in the form of intrusive 



sheets at many points in Merionethshire and Caernarvonshire. 



(2) The occurrence of well-marked contact-metamorphism at the junctions 



of these sheets with the sedimentary rocks. 



(3) The occurrence of brown hornblende, olivine and mica in some of the 



diabases of the Lleyn Peninsula and Anglesea. 



(4) The occurrence of diabase porphyrite and melaphyre (altered plagio- 



clase-augite rock with interstitial matter) in certain localities. 



(1) G. M. Decade II., Vol. VII., 1880. p. 452. 



