Vol. 67.] THE CARBONIFEROUS SUCCESSION IN GOWER. 511 



of others again. The latter, one of which is represented diagram- 

 matical!}- by fig. 5 (p. 510), show clearly that the paths followed by 

 the dolomitization have been the tracts of unrecrystallized ' ground- 

 mass.' PI. XXXIX, taken from the junction of a dolomitized with 

 an undolomitized portion of a pseudobreccia, shows that the dolomite 

 has developed freely in the dense ' ground-mass,' partly as isolated 

 rhombohedra, partly as a band of clustering crystals, but that it has 

 been excluded from the more crystalline ' fragments.' (The dia- 

 gonal cross seen in many of the rhombohedra is probably due to 

 inclusions of the argillaceous impurity in the ' ground-mass.') 



Dolomitized pseudobreccias, such as that figured in the Swansea 

 Memoir, resemble true breccias more closely than do the unaltered 

 rocks, owing to the great contrast in them between the sharply- 

 defined ' fragments ' of dark, fine-grained limestone and the en- 

 veloping pale-grey, rather coarsely-crystalline dolomite. Also, 

 their structure is more conspicuous, whether on unweathered faces, 

 or in cliffs, where they often become cavernous through the 

 dissolution of their ' fragments.' Consequently in Gower, where 

 they abound along the southern coast, they have attracted more 

 attention ; but, as we have now seen, their dolomitization has not 

 been the primary cause of their peculiar structure — it has merely 

 brought into prominence an earlier recrystallization. 



IV. Xote ox Lagoox-Phases and the Origin oe Ra.eiola.rian 

 Cherts [E. E. L. D.]. 



(1) Introduction and Definition. 



By a lagoon-phase is meant a group of rocks, the characters 

 and development of which show that it has been deposited in a coastal 

 area, of wide extent both parallel and at right-angles to the coast, 

 but so extremely shallow as to have been, in effect, isolated from the 

 neighbouring though deeper parts of the sea, and thus to have 

 become the site of peculiar types of sediment and fauna. 1 



A lagoon-area as thus defined is shown in section, at right angles 



1 It has been thought better to adopt the word 'lagoon' and to speak of 

 such an area as that defined above as a lagoon-area, rather than to coin a new 

 expression, because the conditions obtaining in lagoons, as defined in the 

 'Century Dictionary' (London, 1899), must approximate closely to those 

 which it is desired to connote. According to this authority, a lagoon is 

 primarily ' an area of shallow water, or even of marshy laud, bordering on 

 the sea, and usually separated from the region of deeper water outside by a 

 belt of sand or of sand-dunes, more or less changeable in position.' Though 

 there is no evidence that subaerial barriers, such as dunes or coral-reefs, have 

 existed in the case of the lagoon-areas with which we shall deal, possibly because 

 the chances are against their having been preserved, exposed, and correctly inter- 

 preted, they are evidently not essential features ot lagoons. Further, several 

 formations which are lagoon-phases as above defined, such as the Solenhofen 

 Slate, are described by Prof. J. Walther and others as having been deposited in 

 shallow lagoons. 



