TERTIAET MICROZOIC FORMATIONS OF TRINIDAD. 529 



able quantity, though they do not form so large a proportion of the 

 rock as they do in the foraminiferal beds. 



Sample No. 2 is a much harder, tougher, and more coherent rock 

 than the foregoing. Its tinge is slightly more of a greenish-grey 

 and it is less chalky in appearance. Its fracture is more decidedly 

 conchoidal. Uadiolaria and diatomaceae abound in this rock, but 

 foraminifera are not so evident, partly no doubt from its state of 

 aggregation. Coccoliths occur plentifully, and so do the minute 

 stars of Calcaromma calcarea. This rock loses one-half its weight 

 by treatment with acid.^ 



To the account just given of the composition of the microzoic 

 rocks of jSTaparima, I may add a brief description of some" of their 

 physical properties. The soil derived from their decomposition is 

 exceedingly rich and easily worked. As some drawback to this 

 they present difficulties to the road-maker and to the engineer 

 whose business is water-supply. 



[Part of the following is taken from a paper of mine, ' On the 

 Water-bearing capacities of some of the Eock-formations of Trini- 

 dad,' published in the Trinidad Agricultural Record for 1891.] 

 When a piece of the foraminiferal rock is placed in water it absorbs 

 it rapidly and soon falls asunder. This substance is capable of 

 union with a large quantity of water, and the water which enters 

 into union with it is given up only to evaporation. Hence springs 

 or streams are never found in these rocks. That portion of the 

 rain which does not penetrate the soil flows ojff at once as flood 

 water, and that part which does penetrate is only removable by 

 the sun's heat. From these properties it follows that the natural 

 soil-roads passing over these rocks become in the wet season the 

 worst quagmires it is possible to imagine. 



Another property of these rocks is attributable to the same 

 facts of physical constitution. During the wet season they become 

 saturated with moisture. This effect is greatest at the surface and 

 diminishes downwards. An increase of bulk is the result of the 

 absorption of so much water. In the dry season the heat of the 

 sun carries off a considerable part of the water. Hence the soil 

 contracts, producing numerous and extensive cracks. These cracks 

 are often several inches wide and run for long distances. It is 

 somewhat difficult to ascertain their depth, but in some cases at 

 least it is 20 feet or more at the end of a long and severe dry 

 season. These cracks get partly filled with fragments broken and 

 detached from the sides. Then, when the wet season comes on 

 again, the rainwater flows freely into the cracks, washing down 

 the debris of the surface and uniting with the loosened and fallen 

 fragments and with the sides of the cavities. The saturated 

 materials now expand and act as wedges, riving and breaking up 

 the rock to a depth of 20 feet more or less according to circum- 

 stances. The repetition of this action year after year causes such 



^ For complete analyses of these Trinidad rocks, the reader should consult 

 the valuable paper published by Messrs. Jukes-Browne and Harrison in the May 

 number of the present volume of this Journal, p. 219. 



