344 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



the latter to the undifferentiated pre-Cambrian. In so doing they have followed 

 only the suggestion of the lithologic descriptions, as there is no evidence on which to 

 assign either division to a particular geologic age. Sievers states : 



The clay slate is mostly thinly laminated, grayish, brownish to reddish, or bluish to black 

 ia color, and frequently has the character of roofing slate. It occurs chiefly in the middle zones 

 of the Cordillera, from 900 to 3,000 meters, and has an extraordinary development. It is a 

 striking fact in the distribution of the clay slate that in the western portion of the mountains 

 it occurs chiefly south of the gneissic chain; elsewhere, in the eastern mountains, it occurs 

 north of the chain. 



Following these general notes are several pages devoted to the local distribution 

 of the clay slate. 



C 30. TRINIDAD. 



The geology of Trinidad was described in 1860 by Wall and Sawkins,*®^ geolo- 

 gists of the Geological Survey of Trinidad. Their work appears to have been so 

 thorough that it is still the basis of our knowledge of the island. They say in part 

 as follows : 



It is proposed to term that series of micaceous slates, sandstones, limestones, and shales 

 which are the constituent strata of the northern httoral range of Trinidad the Caribbean group, 

 since it forms the southern boundary of the Caribbean Sea; and, from reasons to be adduced 

 hereafter, probably the httoral chain of Venezuela, partly composed also of mica slates and 

 associated rocks, may present a continuation of the same formation. 



The central range comprises an indurated formation of lower Cretaceous age and is appar- 

 ently a fragment of an immense series wliich contributes very largely to the Cordillera of northern 

 Venezuela. These deposits are perhaps equivalent to the series of strata observed very exten- 

 sively in New Grenada and other parts of South America, and described as corresponding to the 

 Neocomian horizon. Plowever this may be, till further investigation more fully exposes the 

 relations of all these deposits, it is well to localize their occurrence; and as this formation was 

 first observed on the shores of the guK which separates the island from the main, it may be 

 appropriately associated with the name of that sea, and wiU be described as the older Parian 

 group. 



A considerable proportion of Trinidad is occupied by a series of Tertiary strata, probably 

 of Miocene age, but since the precise equivalency is not yet estabhshed, this series may, for rea- 

 sons similar to those above stated, be conveniently designated as the newer Parian group. * * * 



The most prevalent beds of the [Caribbean] series consist of mica slates, presenting numerous 

 varieties, sometimes appearing as layers of quartzose matter, separated by thin plates of mica; 

 at others, merely as repetitions of micaceous laminss with little associated substance, frequently 

 possessing extreme hardness and often degenerating into soft micaceous shales. There are 

 several localities where peculiar black slates of ambiguous nature occur. On closer examination, 

 . these seem to be also micaceous schists, the plates of that mineral inclosing pelUcles of graphitic 

 matter. 



The mica often possesses a peculiar glossy appearance and presents numerous shades of 

 white, yeUow, green, brown, red, and black. Of these, green occurs most frequently, after 

 which yellow. 



Some of the schists of finer texture are analogous to clay slates but reafly consist of smaller 

 particles of mica, with finely divided feldspathic matter. 



Quartzose slates frequently occur and, what is rarer, ferriferous slates — that is, fohated 

 rocks in which quartz alternates with oxides of iron, characterized sometimes by great regularity, 

 but more generally the ferriferous laminae occur at unequal intervals. 



Of sandstones there are a considerable variety, from fine to coarse grained, some externally 

 soft and others of the hardest consistency. 



