ON THE OLDER EOCKS OF ANGLESEY. 413 



crystals composed of epidote, chlorite, and calcite, with a fibrous mineral 

 elsewhere referred to talc. 



A mass marked ' greenstone ' on the road between Amlwch and Parys 

 Mountain, protruding through the old schists like a neck (229), is another 

 of these diabases. It shows a groundmass entirely composed of 

 secondary epidote, in which there are many insets of augite, some of 

 them idiomorphic, with large masses of leucoxene in the form of skeleton 

 rhombohedra. This last fact, together with the total conversion of the 

 felspathic constituent into epidote, suggests that it is not much younger 

 in date than the rocks amongst which it occurs. 



There are several other masses of diabase which have either not been 

 examined, or have been found so utterly broken down that little of their 

 original structure can be made out. 



f 



CONCLDSIONS. 



The series of rocks above described are so various in their composi- 

 tion and structure that the results of their study cannot be fully stated 

 in a single proposition, but there are many conclusions of which they 

 either afford demonstration or render the probability considerably 

 stronger. It is of course understood that these conclusions refer only 

 to the rocks of Anglesey, so far as this Report is concerned. These may 

 be formulated as follows : — 



1. The use of the paraboloid and binocular microscope throws new 

 and valuable light upon the structure of such rocks. 



2. The amount of alteration in rocks which form part of the same 

 complex may vary very greatly without our being able at present to 

 assign any adequate cause for the difference. 



3. Rocks may become entirely crystalline without the slightest sign 

 of foliation, a mosaic type of structure being produced, in which areas of 

 yielding to tension may be seen. 



4. Fine-grained rocks more easily become entirely crystalline than 

 coarse-grained ones. 



5. In those crystalline rocks which are most clearly connected with 

 unaltered ones the crystal elements are small. 



6. Foliation-cleavage is produced by the orientation of some of the 

 new minerals, the others remaining unaltered. 



7. Fracture-cleavage is produced by larger cracks, which are after- 

 wards filled with mineral matter. 



8. More complete foliation is unaccompanied by cleavage. 



9. Original structures, such as that of lamination, need not be de- 

 stroyed by foliation. 



10. Very few, except fine-grained, rocks are entirely recrystallised so 

 as to lose all trace of original fragments. 



11. Rocks which under pressure have been contorted or cleaved may 

 have their elements orientated in relation to the pressure, and not to the 

 bedding. 



12. In no case is perfect foliation known to obliterate the bedding, 

 though there are foliated rocks whose original bedding is not certainly 

 known. 



13. The orientation of the minerals in a rock is of various kinds — 

 quincuncial, linear, laminar, elemental, and confused — some of which are 



