41 4 Alfred Earker — Some Anglesey Dykes. 



size are formed by contact alteration is evidently connected with 

 this capacity of tlie mineral for including in its crystals a large 

 quantity of foreign matter. The same remark applies to disthene 

 and chiastolite. 



The analcime crystals, where they are best developed, show the 

 faces of the trapezohedron (211), and have a concentric zonary 

 structure. The specimens were analyzed by Gumming, who pro- 

 nounced them to be " analcime with excess of iron." His figures, 

 however, differ widely from published analyses of the mineral in 

 question, and can only be reconciled by supposing the analcime to 

 be partially changed into prehnite, a mode of alteration known to 

 take place occasionally.^ Assuming the iron to be present as ferric 

 oxide, and replacing it by alumina, Cumming's analysis is represented 

 by column I. below. The second column gives tlae composition of 

 analcime calculated from the formula H^ Naj Alg Si4 O14 ; the third 

 prehnite from the formula H2 Gag Alj Sij Oi2. Column IV. is the 

 mean of II. and HI. 



99 100-0 99-9 99-95 



It will be seen that the Plas Newydd mineral agrees fairly well 

 with the composition of analcime half-converted into prehnite. That 

 such a mineral as analcime should be formed as a true contact- 

 alteration product appears at first very improbable, but the whole 

 process of development can be seen in the specimens, and is precisely 

 similar to that of the garnets. 



Moel-y-don and Plas-Cocli dyhes. — Several dykes, one forty feet in 

 width, were noted by Henslow at Moel-j^-don, opiDOsite what is now 

 Port Dinorwic. These are not marked by the Geological Survey, 

 and I have failed to find any exposures on the shore. Others occur 

 inland on about the same line, near Plas-Goch. 



[563.] Amygdaloidal dolerite from one of the minor dykes, south 

 of Moel-y-don : a medium -grained ophitic dolerite, with nuinerous 

 sphei'ical cavities, averaging 0-1 inch in diameter, filled with 

 secondary minerals, and others larger and more irregular in shape. 

 Under the niicroscope the dominant felspars are seen in elongated 

 sections with ragged ends due to some of the twin-lamellse project- 

 ing beyond others. Between crossed Nicols they show for the most 

 part finely repeated albite-twinning, and some zonary shading. 

 Some sections perpendicular to the twin-plane give extinction- 

 angles up to about 38°, indicating anorthite. The smallest crystals 

 are once twinned, and in several places have a radiate arrangement 

 about a centre. Besides these earlier-formed felspars, there is a 

 second generation, less abundant and, judging roughly from their 



1 Elum., " Pseudomorpliosen," p. 100, etc. 



