NEIGHBOURHOOD OF SARN, CAERNARVONSHIRE. 457 



felspar aud earlier than the hornblende. Another portion seems to 

 be of secondary origin, and connected with a special mode of 

 alteration of the hornblende. In some cases this mineral, as a first 

 stage, takes on a lamellar structure parallel to the orthopinacoid, 

 and from this the biotite-flakes are developed*. More commonly 

 the mica is developed on the cleavage-planes of the hornblende. It 

 is possible that this should be regarded as an original intergrowth 

 of the two minerals, the law of association being that given by 

 Eosenbusch and others. 



The ordinary secondary products from the felspar, hornblende, 

 and biotite demand no special notice. We may remark, however, 

 the presence in some slides of a rather pale-green actinolite in 

 blade-like crystals with a pinacoidal cleavage. 



The hornblende-picrite presents some peculiarities of structure 

 due to the arrangement of its constituent minerals, and these 

 variations are much more striking in the field than in hand- 

 specimens or thin sections. The smooth surface of boulders, both 

 of hornblende-diabase and hornblende-picrite on the beach below 

 Mynydd Penarfynydd and at Y Graig-ddu, often show a curious 

 mottled aspect, which consists in a mingling of dark and light 

 patches or a separation of white spots on a black ground. This 

 kind of aggregation of the several constituents produces on the 

 rock-surfaces in situ a pitted or honeycombed appearance due to 

 differential weathering. When the component minerals are more 

 evenly distributed, the rock either weathers into compact blocks, or 

 in some parts of the picrite-mass shows a fluted or grooved aspect. 

 This last-mentioned appearance seems to be caused by the difference 

 between bands alternately rich and poor in olivine, a character 

 noted by Eeusch in the saussurite-gabbros of the Bergen districtf. 

 There are even distinct dark bands less than an inch in width, 

 which under the microscope appear as veins entirely composed of 

 partially serpentinized olivine with much secondary magnetite. 



The tine section of hornblende-picrite below the trigonometrical 

 signal-staff on Mynydd Penarfynydd shows very clearly the strati- 

 form alternation of the honeycombed and the compact and fluted 

 varieties. The former is in greatest force in the middle half of the 

 section. The picrite often contains coarse segregation-veins similar 

 to those of the hornblende-diabase. 



The intrusive nature of the basic rocks of this district has never 

 been disputed. Wherever the Arenig strata are seen in the vicinity 

 of the eruptive rocks, their contact- alteration is very evident. The 

 shales become hard and flaggy, losing their finely laminated structure 

 and changing colour, while grey spots and yellowish-grey granular 

 streaks make their appearance. 



The Mynydd Penarfynydd mass, exhibiting, as it does, the picrite 

 as well as the hornblende-diabase, may conveniently be considered 

 first. The boundary between these two rocks is a very definite one. 



* This is well exhibited also in the hornblende-picrite of Schriesheim, where, 

 however, both hornblende and mica are bleached. 



t Fossilien-fiihrenden krjstallinischen Scliiefer, &c., 1883. 



