216 ORIGIN OP THE ROCKS AND SCENERY OP NORTH WALES. 



The one from the north side of the lake ia compaot, and of a 

 dull grey colour. It contains a number of small quartz crystals and 

 grains which give it a porphyritic aspect. Under the microscope the 

 grey matrix, when viewed by transmitted light, is seen to be of a dirty 

 brown colour, and to present rather a mottled appearance, the mottling 

 being due mainly to small specks of no definite form, but also, to some 

 extent, to fine needle-like crystals ; almost clear spaces may occasionally 

 be recognised between the mottled spaces. When a comparatively low 

 magnifying power is used, a very peculiar and characteristic structure 

 may be recognised in the matrix. The specks which produce what I 

 have termed the mottled appearance, are seen to follow a linear, or perhaps 

 I ought rather to say, a curvilinear direction. Thus, whenever they 

 approach one of the quartz grains, they bend round, and do not end 

 abruptly against it, as we might at first expect them to do. Under the 

 microscope the quartz grains appear as transparent glass, traversed here 

 and there by cracks and lines of minute cavities. Felspar crystals are 

 also seen to be present in the mass. 



The specimen from Cwm-y-glo is darker in colour, and contains fewer 

 quartz grains. Felspar crystals are, however, extremely abundant. In 

 hand specimens the rock shows a streaky appearance, and also a peculiar 

 platy — one may almost say slaty structure. 



Under the microscope the curvilinear aspect previously described is 

 seen to be developed to a remarkable degree, and the mind almost 

 instinctively looks upon the fragments of felspar and quartz as having 

 been carried along in a current which has solidified in the act of flowing. 



This I consider to be the true explanation, for I agree with Professor 

 Bonney that these specimens are really portions of old lava flows, and 

 that the rock masses to which they belong must therefore have been 

 formed at a period when volcanic action existed in the neighbourhood of 

 Llyn Padarn. This conclusion is borne out by an examination of the 

 chemical and mineralogical composition, and of the microscopic appear- 

 ance just described ; while the peculiar platy structure, so common in 

 many lavas, as Professor Bonney points out, is an additional link which 

 completes the chain of evidence. 



I may state, in passing, that these rocks belong to a class of lavas not 

 found as a rule at a greater distance than ten or fifteen miles from the 

 volcanic vent, and that it is consequently probable that the volcano 

 which emitted these rocks was not further from Llyn Padarn than this 

 distance, although we have no idea as to its exact locality. It will be as 

 well to refer here to one or two other rocks which occur on the shores of 

 Llyn Padarn. 



One of these is a remarkable conglomerate. The term conglomerate 

 is applied to any rock containing water-worn pebbles embedded in a 

 matrix. When the pebbles are small, say about the size of fine shot, the 

 rock becomes a grit. Now the conglomerate of Llyn Padarn varies very 

 much in character in different places. At one spot, near the point of 

 junction with the quartz-felsite, it is composed of a very large number of 



