PRE-CAHBRIAtf ROCKS OF ST. DAVID'S. 293 



in which they should be corrected. The intrusive character and 

 comparatively late origin of the eruptive rocks were deliberately 

 asserted by my colleagues after prolonged examination. Had this 

 view been erroneous, it ought to have been disproved by a detailed 

 review of the evidence on which it was based. I have gone fully into 

 the assertions made by Dr. Hicks himself in regard to the area of St, 

 David's, and have proved them to be untenable. If this is the result 

 of the critical examination of his typical Pre-Cambrian district, over 

 which he has spent most time, I can hardly anticipate that his 

 more rapid traverses elsewhere will, when properly tested, be found 

 to have been more successful. 



Part II. 



Apart from the controversy as to their position in geological 

 chronology, the rocks of St. David's present features of interest and 

 importance not only in the palaeozoic history of Britain, but in regard 

 to general theoretical questions. They include, for example, perhaps 

 the oldest group of lavas and tuffs the relative date of which is known. 

 They have been subjected to a process of metamorphism which has 

 affected only certain beds or kinds of rock. They have been pene- 

 trated by masses of granite and quartz porphyry, round which 

 another kind of metamorphism has been manifested. At a later 

 period they have been injected with diabase dykes, which are 

 specially abundant in the central boss of granite. Evidently, there- 

 fore, they offer much material for study, and especially in regard to 

 the two great geological problems of vulcanism and metamorphism. 

 Though my original design included no more than the examination 

 necessary to satisfy me regarding the disputed questions of the 

 geological age and structure of the St. David's district, it naturally 

 led to many observations of more than merely local interest. As 

 contributions towards a more exhaustive memoir, it may be useful 

 to collect these observations made in the field with my colleague 

 Mr. Peach, and subsequently extended with Mr. Topley. I shall 

 embody with them the conclusions to which subsequent reflection 

 on the subject has led me. One of the most laborious parts of the 

 research has been the microscopic investigation of the rocks collected 

 by us at St. David's. I have studied upwards of one hundred slices 

 of these rocks prepared for the microscope, and have had a large 

 additional number cut from rocks of other regions for the purpose of 

 comparison*. The results of the study are included in the following 

 pages. It is gratifying to be able to state at the outset that, in so far 

 as investigations among the rocks of St. David" s have been published, 

 I can confirm generally the descriptions given of the microscopic 

 structure of these rocks by Professors Bonney and Judd, Mr. Davies 

 and Mr. Tawney. But of the larger number of the rocks no account. 

 as far as I am aware, has yet been given. I am also glad to record 

 that the examination of the microscopic structure of the rocks af- 



* These slices were prepared, in the petrological laboratories of the Geological 

 Survev in London and Edinburgh, by A. Macconochie, R. Lunn, and J. Rhodes. 

 Q.J.G.S. jN t o. 155. z 



