130 ME. J. ROMANES ON THE [feb. I912, 



ground-mass has undergone very considerable alteration and 

 become completely devitrified. Dr. Bonney very kindly examined 

 this rock, and has pointed out the strong resemblance which this 

 ground-mass bears to the mineral pseudophite described by both 

 Dr. TealP and the late J. D. Dana" as forming a special alteration- 

 product of felspar. The residual glass had, therefore, probably the 

 composition of felspar or may even have largely consisted of minute 

 felspar-crystals. 



What the relations of this limburgite are to the ManzauiUa 

 Series it is impossible as yet to say, but I strongly suspect that it 

 belongs to the older limestone series and has no connexion what- 

 ever with the andesitic lavas of Avangares. 



Passing on now to the actual coast-line, the stratified rocks are 

 extremely well exposed at Manzanilla, both on the foreshore and 

 as low cliffs. They differ markedly from those at Las Juntas, as 

 regards both texture and components. The beds crop out roughly 

 parallel to the coast, strike east 20° south, and dip 65° north- 

 wards ; they vary from coarse conglomerates to fine greenish ashes. 

 The conglomerate contains a large and varied collection of boulders, 

 generally subanguiar to rounded, and it is at once noticed, both in 

 these and in the finer-grained types, that non-volcanic material 

 plays a much more important part than in the ashes at Las Juntas. 



In the conglomerates boulders of limestone are common, and this 

 limestone is very similar indeed to that collected in situ farther 

 inland. The main difference is that in the boulders the structure 

 is slightly more crystalline, and the rock contains more numerous 

 foraminifera and calcareous algse, although these are unfortunately 

 rather badly preserved. I am strongly of opinion, however, that 

 the two limestones belong to the same series, and that during the 

 deposition of the Manzanilla Beds hills of the limestone were 

 undergoing denudation. Smaller pebbles of jaspers and cherts 

 of various colours form a common constituent. A curious type is 

 a dark compact rock, which has the appearance of a quartzite, but 

 under the microscope strongly suggests a silicified limestone. To- 

 gether with the typical augite-andesites there occurs a coarse 

 ophitic hornblende-dolerite. 



Turning now to the finer-grained deposits, these are found to 

 vary from an extremely hard compact breccia (see PI. IX, fig. 4) to 

 a fine ashy mudstone. They are composed of subanguiar fragments 

 of many different rocks, set in a matrix partly siliceous but consisting 

 mainly of recrystallized calcite. Yolcanic debris is not plentiful, 

 some pieces of andesite and crystals of felspar and augite being all 

 that can be seen. The commonest component is chert, and in manj- 

 cases traces of radiolaria are distinctly visible in it. Quartz is but 

 sparingly present. The rocks at Manzanilla contain an undoubted 

 marine fauna : foraminifera! remains include Glohigerina and a larger 



1 'British Petrography ' 1888, p. 107. 



- 'System of Mineralogy ' 6th ed. (1892) p. 652. 



