Correspondence — Dr. Henry Hicks. 143 



abundant. Indeed, throughout the whole group there is a schistose 

 character developed, so that in consequence the most massive portions 

 are found to be utterly worthless for dressing, either for building or 

 paving purposes. This fact, though a species of rough evidence, is 

 found to be very valuable in distinguishing many of the metamorphic 

 rocks from those of igneous origin. In the latter the even admixture 

 of the constituents and the regular crystallization enable them to be 

 readily dressed in blocks, whilst the former, except where they 

 consist of limestone or such-like sediments, are seldom sufficiently 

 even in character through any thickness for this purpose. Hence 

 the intrusive granites, greenstones, and various lava flows are 

 frequently used for building and paving purposes, but the metamor- 

 phic rocks but seldom. The term gneissic or schistose may certainly 

 be applied to the St. Davids Dimetian rocks throughout, but perhaps 

 more especially to the middle portion of the group, seen in the 

 valley between the Camp and Ponthclaish. Here tlie beds are 

 usually less massive than at the base, or in the upper or more 

 quartzose portion, and on very slight weathering the foliated 

 character is very marked. Thin lines of nearly pure white felspar 

 are also common, and a tolerably clear gneissic appearance ex- 

 hibited. 



It must not be expected that in these attempts at correlation 

 anything like an absolute identity in character can be found in 

 different areas. Certain general resemblances in mineral character, 

 combined with the physical evidences indicative of contemporaneous 

 deposition, are all we can expect, especially if, as I presume, I may 

 take for granted, that most will now allow that these metamorphic 

 rocks must have had at first an aqueous origin, and were deposited 

 in successive layers of various materials like the alternating sedi- 

 ments found in more recent groups. If we examine closely any of 

 the unaltered groups, capable of being correlated by their fossils, we 

 readily recognize some general mineral resemblances over very con- 

 siderable areas, and Mr. Marr has particularly referred to some of 

 these in his very excellent and highly suggestive paper. But there 

 are, on the other hand, many minor differences, and this is especially 

 the case Avhere the sediments have been deposited in rather shallow 

 water. For instance, the Harlech group at St. Davids and in the 

 Harlech mountains is mainly composed of sandstones, whilst in 

 Carnarvonshire it consists chiefly of slates, and in the north-west of 

 Scotland of conglomerate and grits. Now if we suppose, as I 

 believe was the case, that the Dimetian group was chiefly deposited 

 in shallow water, the differences so well marked in the unaltered 

 rocks of the Harlech group are exactly those which would, under 

 the influence of metamorphisra, produce a massive granitoidite atone 

 place, a quartzose gneiss or micaceous schist at another, and yet a 

 general resemblance indicating the prevalence of tolerably similar 

 physical conditions at the time would be retained in the group in 

 each area. 



On these considerations I think Mr. Marr was fully justified in 

 classing the quartzose gneisses of Bohemia with the Dimetian rather 



