ON THE OLDER ROCKS OF ANGLESEY. 371 



grapliy, the original rocks may be classiSed as follows : (1) rocks composed 

 of uniformly small elements ; (2) rocks of larger grain ; (3) rocks with 

 larger fragments imbedded in a finer matrix ; (4) laminated rocks. 



In the study of these we have to determine, as far as possible, the 

 criteria by which we may distinguish — (1) original fine dnst from the 

 results of crushing or decomposition; (2) fragments of minerals obtained 

 from earlier rocks from crystals formed in situ, and from fragments pro- 

 duced by crushing ; (3) lamination from foliation. 



In determining these points it will be well to lay down beforehand 

 the principles of interpretation adopted, most of which are, of course, 

 well known, and only need to be definitely formulated. In addition, 

 however, to ordinary methods, a plan of observation, not as j-et, so 

 far as I am aware, applied to the microscopic sections of rocks, has 

 been found of great value — viz., the use of a paraboloid with a binocular 

 microscope. The appearance of a rook, when examined in this way, is 

 of course very different to that between crossed Nicols, or even by plain 

 transmitted light ; but the structure is sometimes shown with a clearness 

 which is quite astonishing. Everything in the interior capable of reflect- 

 ing the light is brought out brilliantly, and the perspective of the bino- 

 cular shows the true relative position of such objects. If a rock contains 

 fragments in the midst of authigenetic elements, it will very seldom 

 happen that the union of the two will be complete, and thus the boundary 

 of the fragment will be marked by a reflecting border ; whereas the in- 

 dividual authigenetic elements being in optical contact the light passes 

 through unbroken. The natural shade of colour, both of opaque and 

 transparent objects, is also well brought out by this method ; and the 

 special characters of the cleavage of various minerals is indicated by the 

 reflection of the light from their surface. The fine particles of dust are 

 very clearly seen, whether isolated or in lines, and by the peculiar glist- 

 ening they may be distinguished, however minute, as crystalline or 

 amorphous. The characters of individual minerals as seen in this way 

 are not very describable, but when once the general aspect has been 

 seized, the minerals are easily recognised and their presence is imme- 

 diately revealed. 



The distinction between original dust and that produced by crushing 

 or decomposition is not perhaps always very clear ; but if there has been 

 crushing it will seldom, if ever, happen that none of the other signs, to 

 be hereafter described, shall be present. If the dust be aggregated in 

 the forms assumed by authigenetic elements, either such as they had at 

 first, or such as may have been superinduced, it may be assumed to be 

 due to the decomposition of such. But when it is scattered about irregu- 

 larly, or lies in loose bands of indeterminate form, there can be little 

 reason for considering it as anything but original. 



The distinction of authigenetic elements from fragments when both 

 are crystalline is not in all cases possible. Both may be so small as to 

 present few characters for discrimination, or the metamorphosis may be 

 carried so far as to absorb the boundaries of the fragment, and make the 

 outside practically authigenetic. In general, however, we may rely upon 

 the following critei'ia. A fragment in an altered rock being less easily 

 absorbed, according to its size, will generally be a larger isolated element 

 m the midst of a minuter groundmass. Authigenetic elements being 

 formed from a magma which is capable of motion and mixing, will 

 generally be uniformly distributed throughout the mass ; hence a stray 



B B 2 



