part 3] LOWER PALAEOZOIC OF ARTHOG-DOLGELLET. 281 



the Lower Acid Series. Moreover, the intrusions grow larger, when 

 followed from west to east, roughly in proportion as the Lower 

 Acid Series itself thickens when traced in the same direction. 



The intrusions behave generally as sills, which (even when quite 

 thin) often maintain themselves at the same stratigraphical horizons 

 over long distances. Occasionally, however, the intrusions grad- 

 ually transgress for a time on to a lower horizon, but eventually 

 resume their former position. This is illustrated by the Grelli- 

 llwyd Cae-Einion sill, and by the intrusions near Arthog. In such 

 cases it is sometimes found that the sedimentary rocks are affected 

 by shallow north -and- south anticlinal folds, which have also affected 

 the intrusive rocks, although not to the same extent as the stratified 

 rocks. Further reference will be made to this point (see p. 307). 



But, although the intrusions generally behave as sills, there is 

 at times a strong tendency for their lower boundaries to transgress 

 northwards on to lower stratigraphical levels. As a result, highly- 

 inclined sedimentary rocks are often concealed from view by a 

 more or less horizontal cake of diabase, and it frequently appears, 

 therefore, as if the diabases are more numerous, or have a wider 

 spread, on the higher ground. Instances of this peculiar relation 

 between intrusive and stratified rocks have already been described 

 by Mr. Lake & Prof. Reynolds in the case of the diabases near 

 Dolgelley, 1 and in considerable detail in the case of the great mass 

 of Mynydd-y-Gader, 3 the largest diabase intrusion in the district, 

 which probably represents merely a faulted repetition of the occur- 

 rences nearer Dolgelley. Similar relations hold for the westward 

 continuations of the Dolgelley sills ; but, in view of the accounts 

 quoted above, there is no need to enter here into detail regarding 

 particular cases. 



Petrologically, various types may be distinguished by their 

 texture and quartz-content ; but we have not, so far, determined 

 any direct relationship between these types and the stratigraphical 

 levels at which they occur. It seems probable, therefore, that all 

 the diabases belong to a single phase of igneous activity, and that 

 all are approximately of one and the same age. The actual date 

 of the intrusions will be discussed later (p. 304). 



(b) Petrology. — All the varieties that may be distinguished 

 by texture and mineral composition belong to types which have a 

 wide distribution among the Ordovician rocks of Wales. Since 

 numerous examples of such rocks have been described from other 

 localities, no good purpose would be served by entering upon a 

 detailed description at this point. Accordingly, only the main 

 features need be noted. 



All the true diabases are of simple mineralogical composition, 

 the main differences being those of texture. Neither olivine nor 

 rhombic pyroxene has been discovered, the essential minerals being 

 plagioclase and augite with their respective alteration-products. 



1 Q. J. G. S. vol. lii (1896) p. 517. 



2 Ibid. vol. hcviii (1912) p. 349. 



